US20050281298A1

US20050281298A1 - Analog external cavity laser

Info

Publication number: US20050281298A1
Application number: US11/097,745
Authority: US
Inventors: Vladimir Kupershmidt; Frans Kusnadi; John Major; Sabeur Siala
Original assignee: K2 Optronics Inc
Current assignee: Emcore Corp; K2 Optronics Inc
Priority date: 2004-04-02
Filing date: 2005-04-01
Publication date: 2005-12-22
Also published as: US20080219304A1; US20050220458A1; US7548567B2

Abstract

The present invention relates to the analog external cavity lasers (ECLs) including designs, materials, methods of manufacturing and methods of use for such ECLs and packages for such ECLS. Numerous criteria are presented that lead to improved cost/performance for ECLs and for systems incorporating such ECLs.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from provisional patent application Ser. No. 60/558,927 filed Apr. 2, 2004, and provisional patent application Ser. No. 60/562,762, filed Apr. 16, 2004, and provisional patent application Ser. No. 60/638,679 filed Dec. 23, 2004, pursuant to one or more of 35 U.S.C. § 119, § 120, § 365. The entire contents of all cited provisional patent applications are incorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

1. Field of Invention
This invention relates generally to the field of laser light sources and, more particularly, to external cavity laser light sources.
2. Description of the Prior Art
Directly modulated distributed feedback (DFB) laser diodes are widely used in many applications, including the transmission of multiple channels of analog modulated signals as typically used for analog broadcast, digital simulcast and narrowcast (QAM-format) in cable television (CATV) and telecom networks for video, data and voice-over-IP distribution. Other applications of DFB laser diodes (or simply “DFBs” for economy of language) include the transmission of radio frequency (RF) signals over optical fibers (“RF-over-fiber”) in which RF signals are transmitted over a single strand of optical fiber, typically employing the widely-deployed standard single-mode fiber (SMF) for use at a wavelength of approximately 1310 nm (1310 nanometers). These applications are by way of illustration and not limitation as other applications for DFB laser diodes exist and are being continuously developed.
For CATV and RF-over-fiber applications, the desired system performance has been largely achieved at the wavelength of 1310 nm due in important part to the use of lasers and other components that generate and maintain acceptably low levels of analog distortion. Such analog distortion is typically represented by various numerical parameters such as composite second order distortion (CSO or Inter-Modulation Distortion 2nd Order—IMD2 or 2nd order Intercept Point-IP2) and composite triple beat distortion (CTB or IMD3 or IP3). The desired low levels of analog distortion are typically achieved and maintained through a combination of low distortion DFB lasers and very low (effectively zero) dispersion at 1310 nm in the SMF that is employed. However, there are large potential benefits to be realized by building CATV systems, video and data transmission systems, and RF-over-fiber systems to operate using the wavelength band around 1550 nm. The potential benefits arise in part from the availability of optical amplification means (such as erbium-doped-fiber amplifiers, EDFA) and the ability to take advantage of wavelength division multiplexing (WDM) technology. However, widespread deployment of 1550 nm networks for CATV and wireless distribution has been hampered by several factors, including the chirp produced by direct-modulated DFB lasers. Chirp adversely interacts with the non-zero dispersion in standard SMFs around the 1550 nm band andthe gain slope in typical EDFAs to severely limit system performance as discussed, for example, by E. Bergmann et al., “Dispersion-Induced Composite Second-Order Distortion at 1.5 μm,” IEEE Photonics Tech. Lett., Vol. 3, No. 1, pp. 59-61 (Jan. 1991).
In addition, the deployment of new and more efficient modulation schemes has been delayed due to as yet unmet requirements for higher performing, lower cost optical transmitters. For instance, quadrature-phase-shift-keying (QPSK) modulation techniques can potentially double the system's bit rate of transmission without an increase in the bandwidth required of the electrical components of the system. Also, QPSK modulation techniques can potentially provide a more compact spectrum at the same system bit rate than is possible with binary modulation. However, utilizing QPSK techniques typically requires low cost laser sources having narrower linewidth than typical DFBs that are currently commercially available (see, for example, S. Norimatsu, et al. “An 8 Gb/s QPSK Optical Homodyne Detection Experiment Using External-Cavity Laser Diodes”, IEEE Photonics Tech. Lett., Vol. 4, No. 7, pp. 765-767, July 1992.
Among the major barriers to the wider use of direct-modulated DFBs around the 1550 nm band is the high chirp and the relatively high intrinsic distortion of the solitary DFB laser. Such effects arise chiefly because of three physical phenomena (i) spatial hole burning effect, (ii) leakage current, and (iii) intrinsic nonlinear response.
Another limitation of direct-modulated DFBs operating near the 1550 nm band is the relatively large linewidth, of the order of 1 MHz. Such a linewidth limits the Relative Intensity Noise of the laser (RIN) which in turn limits the transmission carrier-to-noise ratio (CNR). Such effects lead to limited transmission performance under high channel loadings and for longer transmission distances.
Multiple approaches have been proposed to address these issues, but they generally have technical and/or cost drawbacks. These solutions include (a) The use of dispersion compensators, which are usually expensive, complex, and cumbersome, typically requiring customization of each fiber span. (b) The use of externally-modulated continuous wave (CW) DFBs coupled to a Mach-Zehnder modulator. This combination can exhibit practically zero chirp in some circumstances, but the high cost makes this solution over-engineered and too expensive for all but a few specialized, typically low volume, applications. (c) The use of electro-absorption modulated lasers (EMLs) which suffer from narrow operating margins, the requirement for complex predistortion circuitry to reduce the relatively large intrinsic harmonic distortion, and the low carrier-to-noise ratio (CNR) (due to the relatively low optical output power).
The high cost of the optical transmitter solutions mentioned above has proven to be an important factor leading to the development of a number of techniques that enhance the performance of direct-modulated DFBs. These approaches generally include one or a combination of the following techniques: (i) Electronic predistortion techniques to correct for degradation in second order distortion (for example, see U.S. Pat. Nos. 5,436,749; 4,992,754; 5,227,736) and to correct for third order distortion (for example, U.S. Pat. No. 5,172,068). The main drawbacks of such techniques include the added expense and the need for customization in manufacturing to accommodate different levels of distortion correction. (ii) Optical injection locking techniques, whereby light from a master laser is injected into a slave laser whose output is then locked to the master laser. However, this method has had very limited commercial success because of its high cost and complexity (for example, see H. Sung et al., “Dependence of Semiconductor Laser Intermodulation Distortions on Fiber Length and its Reduction by Optical Injection Locking,” Conference Proceedings, Paper WE2 10, p. 186-200). (iii) Optical linearization techniques whereby a DFB is optically enhanced using an external optical element (for example, see U.S. Pat. No. 6,538,789).
Thus, a need exists in the art for an improved distributed Bragg reflector laser diode having low analog distortion and/or that can be made available at relatively low cost.

SUMMARY OF THE INVENTION

The present invention relates to the design, packaging, and manufacturing of direct-modulated analog external cavity lasers (ECLs) having improved cost-performance ratio for transmission of analog and semi-analog (such as quadrature amplitude modulated (QAM)) signals, particularly for broadcast, digital simulcastand narrowcast (QAM) applications.
Furthermore, some embodiments of the present invention relate to the design of direct-modulated analog ECLs with simultaneously (i) controlling chirp (from extremely low to high in magnitude); (ii) low intermodulation distortions (second- and third orders); and low Relative Intensity Noise (RIN) which provide high performance and low cost devices, methods and/or systems for transmission of analog signals in the 1550 wavelength range.
The present invention further relates to and includes packaging design and packaging criteria (particularly 14-pin butterfly packaging common in the industry) and with the external laser cavity implemented using a fiber Bragg grating element terminated with an integrating high coupling lens and light source (Fabry-Perot (FP) chip), all mounted on the same solid substrate. Such an approach provides long-term package stability that is particularly advantageous in analog transmission systems.
The present invention further relates to and includes designs and methods for increasing the coupling efficiency within the cavity between the FP chip and the grating element while reducing unwanted reflectivity within the cavity of the analog ECL.
The present invention further relates to methods for reducing the amount of reflected light coupling back into cavity of the analog ECL. Suppressing reflected optical energy from coupling back into the cavity is paramount to the performance of the analog ECL in transmission systems. Methods for achieving high level of suppression of reflective light include the incorporation of an in-line optical isolator in the pigtail of the analog ECL and/or the appropriate design of the reflectivity of the reflective surface of the external cavity.
The present invention relates further to the enhancement and use of the so-called distortion dip in analog ECLs by properly designing the reflective external reflective element and by appropriate temperature control of the ECL.
The present invention relates further to the design and implementation of both intra-cavity and extra-cavity methods for suppressing Stimulated Brillouin Scattering (SBS) which, if not adequately reduced, can severely limit the amount of optical power that can be launched into the fiber.
The present invention relates further to the design and implementation of techniques for tuning and stabilizing the emission wavelength of the analog ECL's output power to be within industry standards for dense wavelength division multiplexed (DWDM) systems, without sacrificing desirable distortion, chirp, and RIN properties of the analog ECL.
The present invention relates further to methods for reducing the so-called frequency tilt, which is manifested by higher level of before-link and after-link distortion affecting the higher frequency channels launched into the fiber.
The present invention relates further to methods for designing analog ECLs so as to enable the successful incorporation of known predistortion and Electronic Dispersion Compensation (EDC) technologies to further improve the distortion and reach performance of transmitters utilizing these analog ECLs.
The present invention relates further to methods for fabricating analog ECLs using Bragg gratings created inside the cores of various optical fibers, including standard non-dispersion shifted single mode fiber (such as Corning SMF-28), dispersion-shifted single mode fiber, polarization-maintaining single mode fiber, and graded and step-index multimode fiber.
These and other advantages apparent to those skilled in the art are achieved in accordance with various embodiments of the present invention as described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings herein are not to scale and the depictions of relative sizes and scale of components within a drawing and between drawings are schematic and also not to scale.
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.
The techniques of the present invention can readily be understood by considering the following detailed description in conjunction with the following drawings, in which:
FIG. 1 depicts a typical ECL configuration in block diagram form.
FIG. 2 is a schematic depiction of an ECL constructed with: (a) a fiber Bragg grating, and (b) a Bragg grating written in a waveguide on a PLC.
FIG. 3 is a schematic depiction of an external cavity laser.
FIG. 4 is a graphical depiction of ECL emission wavelength vs TEC temperature denoting mode hop regions.
FIG. 5 is a graphical depiction of the spectral response of an ECL near a mode hop region, also depicting a jump in peak wavelength as the temperature is changed from 20.2 deg. C. to 19.8 deg. C.
FIG. 6 is a graphical depiction of slope efficiency vs bias current, that is the first derivative of the L-I curve of the ECL.
FIG. 7 is a schematic depiction of an ECL having the chip and grating assembled on the same substrate to ensure improved stability. Two attachment points can maintain the FBG section under a controlled stress (compression or tension) in order to achieve improved performance under direct analog modulation.
FIG. 8 is a graphical depiction of a distortion profile of one ECL embodiment including a distortion dip.
FIG. 9 is a graphical depiction of second order distortion (e.g. IMD2) and chirp as functions of temperature range between mode hops for one embodiment of FBG design for an analog ECL. The figure depicts the ability to select a desired chirp level by the appropriate selection of the operating temperature, that is the TEC temperature. Experimental results are depicted.
FIG. 10 is a graphical depiction of second order distortion (e.g., IMD2) (left vertical axis) and chirp (right vertical axis) as functions of the temperature range between mode hops for an embodiment of the FBG design (the FBG with knee profile) for an analog ECL for the example in which the one-sided butterfly profile (distortion dip) is aligned with maximum chirp. The figure depicts results of numerical simulation.
FIG. 11 is a graphical depiction of FBG reflectivity profile as a function of wavelength span. The “knee” spectral profile facilitates the alignment of the OSBDP (distortion dip) with the high chirp region as depicted in FIG. 10.
FIG. 12 are schematic depictions of ECL multi-wavelength arrays including: (a) multiple distinct optical output beams; and, (b) multiple output beams multiplexed into a single strand of fiber or onto one waveguide on a PLC.
FIG. 13 is a schematic depiction of SBS suppression applied to analog ECL using an External Phase Modulator as a Bragg grating written inside a polarization-maintaining fiber (PMF) in the embodiment depicted in this figure.
FIG. 14 is a schematic depiction of an analog ECL with an in-line optical isolator in the pigtail.
FIG. 15 depicts CSO tilt of analog ECL and its improvement using wider bandwidth FBG. CSO tilt is defined here as the difference in CSO at a medium frequency of 314.5 MHz and at a high frequency of 547.5 MHz.
FIG. 16 depicts CSO tilt of analog ECL at high frequency (547.5 MHz) and its improvement using chirped FBG.
FIG. 17 is a schematic depiction of an analog ECL with an integrated optical isolator and optical filter spliced into the pigtail of the analog ECL. The optical filter reduces spontaneous emission.
FIG. 18 is a graphical depiction of attenuation and dispersion vs. wavelength of standard single-mode fiber and dispersion shifted fiber.

DETAILED DESCRIPTION OF THE INVENTION

After considering the following description, those skilled in the art will clearly realize that the teachings of the invention can be readily utilized for the design, fabrication, packaging and/or use of external cavity lasers (ECLs), particularly analog ECLs
The technologies described herein relate to analog external cavity lasers including techniques for designing, packaging and improving the performance of ECLs for use in analog and CATV fiber optic communications systems. This field of application is by way of example and not limitation since systems, techniques, processes, devices and materials described herein can find applications in other fields as well. The analog ECLs described herein are direct-modulated laser sources providing significant advantages compared with other devices and systems for analog optical transmission. These advantages include higher performance in terms of distortion and chirp (for example), at lower costs and with improved design margins.
A typical ECL includes a laser diode chip 200 having one highly reflective face, typically coated with high reflectivity (HR) film 202, and the opposite face transmissive to the light produced by the laser diode but typically coated with an anti-reflective (AR) coating 201. In combination with an optical feedback element 103 an optical transfer function, F(λ) is formed as depicted in FIG. 1. The optical transfer function, F(λ), is usually a grating, sometimes referred to as a Bragg grating, and behaves effectively as both a mirror and a filter, reflecting some light incident thereon, 104, back to the laser diode chip 200 while transmitting another portion of the light 105 to produce the output of the laser. The laser light produced by the ECL bounces back and forth between the highly reflective (HR) side of the laser diode 202 (that is, the side opposite the AR-coating) and the grating 103, forming thereby the laser cavity.
Pursuant to some embodiments, the present invention includes an external cavity laser (ECL) in which the grating is created inside an optical fiber, (and thus often called a fiber Bragg grating (FBG)), depicted as 203 illustrated in FIG. 2(a). In other embodiments, the grating can be created inside planar waveguides on planar lightwave circuits (PLC) as illustrated as 203 in FIG. 2(b). In yet other ECL embodiments, the optical transfer function can comprise a bulk grating, an acousto-optic filter, an etalon-type thick or thin film filter, built in glass, semiconductor or other material. In other embodiments, the grating can be created in semiconductor material (such as indium phosphide, InP). Additional ECL embodiments include the use of a tunable grating wherein the optical transfer function can be tuned using such techniques as spatial movement of the grating (e.g., using micro-electro-mechanical systems (MEMS)) or the application of an RF signal to tune an acousto-optic based optical transfer function, or the application of stress to tune a thin- or thick-film based tunable optical filter, or the use of a tunable grating written in or on a semiconductor chip, among other techniques. Such embodiments may, but need not, include a beam-shaping element between the light-generating chip and the external reflective surface.
The analog ECLs described herein are typically described in connection with an external cavity employing a fiber Bragg grating (FBG) as reflective element. However, this is by way of illustration and not limitation since the techniques described herein can readily be utilized in connection with external cavities making use of other reflective elements, as apparent to those with ordinary skills in the art.
Laser source modules are typically specified for use in practical communication systems (or other applications) in which the laser module can be subject to wide ambient temperature ranges (e.g., −20° C. to +75° C.). Since the operating wavelength is typically a sensitive function of temperature, it is known practice to provide temperature sensing and temperature controlling devices in the module package to maintain a narrower range of operating temperatures (say on the order of 15-35° C.) within the module package environment. A widely used cooling device is a thermoelectric cooler (TEC).
Embodiments of the present invention include analog external cavity lasers advantageously designed and built according to several criteria. The development and employment of such criteria comprise aspects of the present invention. The resulting analog ECLs achieve several advantages in comparison with typical prior art devices.
The packaging of the ECL is an important component of the overall ECL system. Some embodiments of the present invention include design criteria for the ECL packaging. By way of illustration and not limitation, the embodiments described herein relate to an FBG-based analog ECL including a lens integrated at the tip of the fiber containing the FBG.
Analog ECLs pursuant to some embodiments of the present invention include a combination of favorable choices of Fabry-Perot (FP) gain element (or laser diode chip, 200), external cavity design and proper gap adjustment between the optical component containing the grating and the laser diode chip, wherein the gap adjustment affects the optical coupling. Diffraction losses and advantageously detuned loading conditions are also considered in ECL designs pursuant to some embodiments of the present invention. In general, advantageous levels of CSO distortion may be determined by the FP chip performance or by external cavity design having particular amplitude and phase profiles. Therefore, design criteria developed and used pursuant to some embodiments of the present invention typically include certain constraints on the FP chip.
Design and packaging criteria have been developed pursuant to some embodiments of the present invention based on both experimentation and on a computer model (FIG. 3), in 300 to address packaging sensitivity and coupling of the FP chip to the component containing the grating 301. Typical criteria include the following:
The Fabry-Perot gain element advantageously provides a constraint on the active length, (La in FIG. 3) between approximately 300 μm and approximately 600 μm (μm=micrometer=10⁻⁶meter)
The lower limit on the length of the FP chip, La, is determined chiefly by the linearity of the plot of the optical output power of the ECL versus the laser's injection current (usually referred to as the L-I curve) and the adiabatic chirp response of the Fabry-Perot chip in the frequency range of interest (e.g., in the range up to approximately 10-3,000 MHz for CATV and RF-over-fiber applications. This relationship tends to be followed since adiabatic chirp response is typically a function of 1/La. The upper limit of La is chiefly dictated by the temperature span between mode hops which is directly related to FP chip length.
Temperature span between mode hops depends upon many factors, including the length of the FP chip and, less importantly, the relative position of the grating element with respect to the gain chip. As the ECL TEC temperature is changed, mode hops are manifested by an increase in the ECL emission wavelength occurring suddenly after a linear region, in an approximately step-increase fashion as depicted in FIG. 4. The mode hop region exhibits a hysteresis effect, whereby different traces for wavelength versus TEC temperature are obtained depending on whether the temperature is swept in the upward or downward direction. For example, an FBG-based ECL built with an InGaAsP/InP FP chip with an active length of about 500 μm exhibits a temperature interval of about 8° C. between mode hops as shown in FIG. 4. Operating the ECL at or close to the mode hop region is typically disadvantageous because of potential instability of the output of the analog ECL. It is prudent to include a “safety margin” of the temperature operating point away from positions of mode hops of about 1° C. or more. Thus, the available space/margin for ECL analog packaging with a 500 μm chip length is thus reduced to 5-6° C., and for a longer chip, to an even smaller margin. It is therefore an aspect of some embodiments of the present invention to select the length of the gain element chip and of the overall cavity so as to allow an operating temperature window of at least a few deg. C. between mode hops.
The high reflectivity (back facet) coating on the Fabry-Perot gain element chip typically has a reflectivity above about 75-95%, and an AR coating (front facet) with reflectivity less than about 0.05%. Some embodiments of the present invention have the waveguide positioned at an angle (typically about 6-10 deg) from the front facet of the gain element chip thereby relaxing the AR coating specifications for the front facet.
Constraints on the AR coating are typically dictated by the desired level of distortion and available temperature margin, which depend on the magnitude of the AR coating. The temperature separation between mode hops for a 500 μm chip is only about 8° C. and the temperature width of the region of mode hops is very small, approximately 0.15-0.2 deg. C. for an AR less than about 0.05%. If the AR coating is not of sufficiently high quality, for example >0.07%, then the temperature width of the mode hop region increases to about 0.5-0.7 deg. C., caused in part by the increased competition between lasing modes within the hysteresis region. As the ECL temperature is scanned from a stable region through a mode hop region, the modal behavior of the ECL typically changes from single mode with unacceptably low side-mode suppression ratio (SMSR) to multi spectral modes as illustrated in FIG. 5. When the AR coating is less than about 0.05% reflectivity, single mode operation with SMSR larger than about 35 dB could be reached within ±0.2 degree from the boundaries of the mode hop.
External cavity design typically requires a high-performance FP, particularly low leakage current, high slope efficiency, small alpha (α)-factor (less than about 4) and magnitude of adiabatic chirp. One of the indications of a high-efficiency FP chip with low leakage current is a very small change in the differential resistance measured at high bias current compared with its threshold value. Additional requirements on the FP chip arise from the fact that analog ECLs typically require high optical coupling efficiency between the chip and the reflective element of the external cavity. Therefore, it is advantageous to use a type of buried-heterostructure (BH) with a symmetrical far-field beam profile (for example, having beam ellipticity less than about 1.2). Such far-field beam profiles allow the use of fiber-integrated lenses in an FBG-based analog ECL with high coupling efficiency (for example, around 56-70%).
Parameters such as those enumerated herein typically allow practical packaging of ECLs with advantageous amounts of available power.
Another aspect of some embodiments of the present invention relates to achieving low distortion analog ECLs by means of a cavity design that has a linewidth enhancement factor (α-factor) at the wavelength corresponding to the gain peak of the FP chip less than about 4.
In addition to the limited magnitude of the α-factor related to the FP chirp at the gain peak, the distortion performance of analog ECLs pursuant to some embodiments of the present invention benefits from so-called blue detuning. Blue detuning is the amount of offset between the ECLs lasing wavelength from the lower wavelength side (blue side) of the gain peak of the FP chip. Blue detuning leads to a reduction of the α-factor with wavelength, which is advantageous for achieving lower distortion in analog ECLs. Typical amount of blue detuning is within a range of about 20-50 nm from the gain peak. To suppress competition from lasing at the peak of FP gain, it is typically necessary to use a high-performance AR coating, with typical AR coating less than about 0.05%
To reduce levels of distortion, the design of the external cavity should avoid coupling the optical signal from the coated (HR/AR) FP chip to a high-Q resonator. That is, keeping the reflectivity of the grating (R) below about 25% typically leads to favorable cavity design.
Concurrently, if grating reflectivity becomes very small, for example R ˜5%, then the cavity typically becomes unstable as mode competition increases and packaging of such a cavity becomes extremely difficult. Thus, a favorable range for R is approximately 5% <R<25%.
It is advantageous to increase and strive to maximize the coupling efficiency (C) of the light from the coated (HR/AR) FP chip into the grating element, such as a single mode fiber for the case of FBG-based ECL, or a planar waveguide for the case of PLC-based ECL, or any combination of FP chip, coupling optics and external resonator which constitute an external cavity. Maximizing coupling efficiency can be approached by improving the optical beam quality exiting the FP chip and by proper design of the coupling optics as depicted in FIG. 1.
An approximate lower limit on the minimum coupled power is typically dictated by two factors: Constraints on the available power for the analog laser (typically >7 dBm) and the criteria described herein for the coupling efficiency C, which leads to the effective ECL “front facet” reflectivity R_eff=C²*R_max. If C ˜0.7 (for typical practical implementations), and R=15%, then R_eff=7.4% which indicates a weak cavity and high FP mode competition. These considerations again lead us to derive certain conditions on AR coating reflectivity minimization. The upper constraint on coupling efficiency (Cmax) is dictated by the practicality of the coupling optics and sensitivity to the alignment.
Integrated lens design is an important embodiment that provides mechanical and detuned loading stability in the ECL package.
Design of the external cavity (external FBG or grating on PLC substrate) also advantageously follows certain criteria, pursuant to some embodiments of the present invention. Among these is the criterion that, in a single mode operation, SMSR >35 dB.
Other criteria include that the ratio of mode spacing Δλsp (cold cavity when the ECL bias current=Ibias ˜Ith, near the lasing threshold) to the bandwidth (BW) of Bragg grating, is advantageously in the range 0.5<Δλsp/BW <1.3.
Such criteria as described herein tend to avoid mode competition by producing a high level of discrimination between a dominant mode and the secondary modes (that is, generate single mode operation with SMSR >35 dB). At the same time, such criteria can be used as a practical optimization parameter during the assembly of the analog ECL cavity.
Relationships between the passive length of the ECL cavity (Lp in FIG. 3) and the so-called effective length, Leff, of the ECL grating, permit the chirp of the external cavity to be effectively tailored depending on the application. Leff for a so-called weak grating (R less than about 15%) is a function of the grating bandwidth and the reflectivity profile.
For both an external FBG or a Bragg grating on a PLC-platform, the side-lobe suppression of the grating (SLS) advantageously exceeds about 15 dB in order to provide SMSR of the ECL >35 dB within 1° C. temperature interval from mode hop temperature Th.
One example of FBG design pursuant to some embodiments of the present invention is an apodized grating as a practical implementation. Criteria related to the chirp requirements still allow the designer of analog ECLs a choice of certain relationships between passive length and ECL effective length.
Another aspect of the present invention relates to the exploitation and tailoring of the detuned loading in ECL in order to reduce the second and/or third order distortion under direct analog modulation while simultaneously achieving a desired chirp level. In ECL designs pursuant to some embodiments of the present invention, the detuned loading parameter X_DL=(λ-λp)/BW (where λp is peak wavelength of the grating profile and λ is the lasing wavelength of the ECL) plays an important role in the mechanism of ECL distortion (both 2nd order, CSO, and 3^rdorder, CTB). In the assembly of the external cavity of ECLS, such parameters are typically sensitive functions of the gap between the AR-coated facet of the FP chip and the effective reflectivity plane of the grating.
Modeling and analysis of the distortion mechanism and practical experiments show that for essentially any type of reflectivity profile of the optical element F(λ), a unique signature for the dependence of slope efficiency (which is the first order derivative of the L-I curve, shown in FIG. 6) on bias current is obtained, which provides minimum distortion across the temperature range between mode hop positions. Such a signature is attributed to the fact that the threshold current of the ECL and the effective reflectivity of the grating depend on a detuned loading effect, which varies with bias current and temperature. A certain detuned loading condition is advantageously employed in order to achieve reduced distortion and this detuned loading condition gives a certain signature shape of the slope efficiency curve. Reduced and substantially minimum distortion is typically achieved when the slope of the efficiency curve peaks around the threshold current then decreases as the bias current is increased, to reach a minimum substantially at the target optical output power, and then increases again. One aspect of some embodiments of the present invention relates to the use of such signatures in the design, construction and assembly of analog ECLs.
This signature of the slope efficiency curve is typically sensitive to the detuned loading. Thus, to ensure the mechanical stability of the ECL across environmental changes, the chip and the grating of an FBG-based ECL are advantageously assembled on the same substrate, as illustrated in FIG. 7.
One aspect of some embodiments of the present invention relates to actively measuring and monitoring the second order distortion (CSO or IMD2 or IP2) as part of the assembly process of the analog ECL while the external cavity is being aligned. When a desired level of distortion is achieved, the optical feedback element (F(λ) is fixed in place. For example, in case of FBG-based ECL, the FBG is aligned then fixed in place using the attachment points as illustrated in FIG. 7. This technique typically results in improved manufacturing yields and improved uniformity from one ECL device to another. In addition, monitoring the distortion and/or chirp during assembly enables low-cost customization of the device if needed.
With respect to the second order distortion, analog ECL has a unique feature consisting of a region of ECL operating temperature where 2nd and 3rd order distortion is small at all carrier frequencies. This feature is called “one-sided butterfly distortion profile” (OSBDP) or “distortion dip.” This distortion dip typically occurs within a narrow operating temperature range of ΔT approximately 1-2 deg. C., usually occurring close to the mode hop position, as shown in FIG. 8. Analog ECLs pursuant to some embodiments of the present invention typically have CSO that is comparable or better than that offered by DFB CSO performance at any operating temperature, but at significantly lower chirp than DFBs. This makes analog ECL lasers very attractive for broadband and narrowband analog applications (CATV, QAM, RF-over-fiber, among others) and may result in dramatically increased transmission distances and expanded applications such as fiber-to-home/premises (FTTH/P).
Another aspect of some embodiments of the present invention relates to tailoring and tuning the chirp in such a way as to provide an improved, substantially optimum chirp level at the desired distortion performance, where the chirp is high enough to suppress SBS but not too large so as to limit the maximum reach of the transmitted analog signal. Having an ECL with low level of distortion does not automatically guarantee adequate signal performance over the analog fiber link, because low chirp interacts with nonlinear effects in typical optical fibers to make SBS and so-called double Raleigh scattering (interferometric noise) the limiting factors for link performance. Furthermore, existing methods of SBS suppression using RF dithering, which have been developed for DFBs and externally modulated sources, may be inadequate for suppressing SBS in ECL-based transmitters. One approach to addressing the SBS issue with ECL-based transmitters is to increase the chirp level while maintaining low distortion. One embodiment suitable for achieving relatively high chirp analog ECLs while maintaining low distortion and low RIN is to shorten the cavity length by reducing the passive section Lp in FIG. 3.
FIG. 9 (experimental results) and FIG. 10 (simulated computer model results) show two different designs of an analog ECL in which a certain level of chirp can be selected while maintaining low distortion (both 2^ndand 3^rdorder) by operating the ECL at the properly chosen temperature. This invention also relates to exploiting and designing analog ECLs with (i) tunable chirp and, (ii) increased levels of chirp. Tunable chirp allows the user to “dial-in” the chirp level more appropriate for his application. To achieve relatively high chirp (>35 MHz/mA) in analog ECLs while maintaining low distortion, the grating element of the ECL (including but not limited to FBG, waveguide grating in PLC-based ECL, bulk grating, acousto-optic based grating) is designed according to one of the following guidelines: (i) to be a chirped grating; (ii) a grating with a spectral profile referred to as a “knee” spectral profile as depicted in FIG. 11; (iii) a grating with a spectral profile in which chirp is introduced by external means such as gradients created by stress.
Electronic dithering techniques can also be used with ECLs instead of, or in addition to, relatively high levels of chirp. One technique is to apply to the analog ECL input triangular-shaped pulses or a set of tones at relatively high frequency whereby the total chirp is dominated mainly by transient components.
Another aspect of some embodiments of the present invention relates to FBG-based analog ECLs whereby the FBG is assembled and fixed in place under a mechanical stress (either under tension or compression) in order to improve the performance of the ECL for direct analog modulation, and/or to tune the emission wavelength to be on the grid specified by the standardization bodies for wavelength division multiplexing (WDM) systems. Mechanical stress applied to the FBG is used to tailor the distortion, the chirp, and/or the emission wavelength of the analog ECL. As an example, in FIG. 7 the FBG section is maintained under controlled stress by using two attachment joints (such as those resulting from soldering, laser welding, or epoxy).
Another aspect of some embodiments of the present invention relates to design and implementation of analog ECLs so as to have their emission wavelengths centered on, or locked to, or adjacent to, the industry-standardized ITU wavelength grid so that said ECLs can be used in wavelength division multiplexed (WDM) systems whereby multiple independent optical beams at different wavelengths, each carrying different information are launched into one strand of optical fiber. The grating element of typical analog ECLs as described herein can be designed to have a peak wavelength such that, after proper adjustment of ECL TEC temperature for improved chirp and distortion, the emission wavelength of the ECL remains within the ITU wavelength grid tolerances.
Another aspect of the invention relates to design and implementation of an array of analog ECLs using monolithic and/or hybrid integration. Each ECL element in the array emits at a predetermined wavelength and can be independently modulated. The elements in the array may, but need not, share the same temperature controller. A typical embodiment of such an analog ECL array is illustrated in FIG. 12. Examples for implementing such an ECL array include the use of an FBG array or grating in a waveguide array on a PLC. The multi-wavelength output from the array is passed through an optical multiplexer in FIG. 2(b).
Another aspect of some embodiments of the present invention relates to FBG-based analog ECLs whereby the FBG is created inside a polarization-maintaining fiber (PMF). Such an embodiment results in direct analog modulated ECLs having lower noise than those built with standard single mode fiber. In addition, ECLs built with PMF can be used as high quality low-noise CW lasers sources in an externally modulated analog transmitter. In addition to controlled polarization, ECLs built with PMF provides simultaneously narrow linewidth, low relative intensity noise (RIN), and high optical output power, which can then be coupled into the external modulation means.
Another aspect of some embodiments of the present invention relates to direct analog modulated ECLs fabricated using a Bragg grating inside a PMF fiber pigtail which is interfaced with a phase modulator (such as a lithium niobate modulator), as widely used in prior art to suppress SBS in analog systems. This is depicted in FIG. 13.
The present invention further relates to and includes packaging design and packaging criteria (particularly 14-pin butterfly packaging common in the industry) and with the external laser cavity implemented using a fiber Bragg grating element terminated with an integrating high coupling lens and light source (Fabry-Perot (FP) chip), all mounted on the same solid substrate as illustrated in FIG. 7. Such an approach provides long-term package stability that is particularly advantageous in analog transmission systems.
The present invention further relates to and includes designs and methods for increasing the coupling efficiency within the cavity between the FP chip and the grating element while reducing unwanted reflectivity within the cavity of the analog ECL. One embodiment for FBG-based analog ECLs is the use a microlens fabricated at the tip of the fiber containing the grating. Embodiments of such fiber lens include a tapered hemispherically shaped lens and a hyperbolic shaped lens.
The present invention further relates to methods for reducing the amount of reflected light coupling back into cavity of the analog ECL. Suppressing reflected optical energy from coupling back into the cavity is paramount to the performance of the analog ECL in transmission systems. Such methods for achieving high level of suppression of reflected light include the incorporation of an in-line optical isolator in the pigtail as depicted in FIG. 14.
The present invention relates further to the enhancement and use of the distortion dip in analog ECLs by properly designing the reflective external reflective element and the temperature control of the ECL.
The present invention relates further to the design and implementation of intracavity methods for suppressing Stimulated Brillouin Scattering (SBS) which, if not suppressed adequately, can severely limit the amount of optical power that can be launched into the fiber. Intracavity SBS suppression in analog ECLs is based on modulating the effective optical path length of light inside the cavity. Some embodiments of intracavity SBS suppression in analog ECLs include (i) applying a modulated electrical signal to a piezoelectric transducer or an electrostatic element placed adjacent or onto the reflective element (such as FBG); (ii) modulating the position of the reflective elements of the external cavity (iii) superimposing a dithering electrical signal on the gain element chip of the analog ECL.
The present analog ECL relates further to methods for reducing the so-called frequency tilt, which is manifested by higher level of before- and after-link distortion affecting the higher frequency channels launched into the fiber. A method for improving both the overall distortion and frequency tilt of analog ECL is to reduce the dispersive nature of the grating element used as the reflective element in the external cavity. Embodiments of this invention for grating-based ECLs include the use of one or a combination of one or more of the following techniques: (i) Design the grating with a relatively large bandwidth (for example, a FBG with optical profile bandwidth in the range of about 220-450 pm where pm=picometer=10⁻¹²meter) at full width at half maximum (FWHM)), as seen from the experimental data presented in FIG. 15; (ii) Use non-apodized grating as the reflective element of the external cavity; (iii) Use chirped FBG with appropriate gradient direction so as to cancel dispersion effects in the grating, as shown by the experimental data presented in FIG. 16.
The present invention relates further to methods for improving the design of analog ECLs so as to enable the successful incorporation of predistortion and Electronic Dispersion Compensation (EDC) technologies widely described in the prior art to further improve the distortion and reach performance of transmitters utilizing these analog ECLs.
The present invention relates further to methods for building analog ECLs using Bragg gratings created inside the core of various optical fibers including standard non-dispersion shifted single mode fibers (such as Corning SMF-28), dispersion-shifted single mode fibers, polarization-maintaining single mode fibers, and graded and step-index multimode fibers.
The present invention further relates to reducing the amount of spontaneous emission noise from the output beam emitted from the analog ECL. Suppressing the spontaneous emission improves the performance of the transmission especially when cascaded optical amplifiers are present in the link. One embodiment involves the splicing an in-line optical filter (such as a thin film filter) into the analog ECL pigtail. Another embodiment is to integrate the optical filter in the isolator used in the pigtail of the analog ECL as depicted in FIG. 17.
Another aspect of some embodiments of the present invention relates to direct analog modulated ECLs (including but not limited to ECLs based on FBG, grating in PLC waveguide, bulk grating, and acousto-optic grating) whereby the emission wavelength is in the 1310 nm wavelength range. Direct analog-modulated 1310 nm ECLs provide benefits in both cases for the transmission medium: dispersion-shifted fiber and standard non-dispersion shifted fiber. In the case of dispersion-shifted fiber, the low chirp characteristic of ECL improves reach capability and lowers link distortion by reducing the interaction of laser chirp with the non-zero dispersion in the fiber. This is similar to the benefit obtained when 1550 nm ECL are used in standard SMF.
At the 1310 nm wavelength range in standard (non-dispersion shifted) single mode fiber, the fiber dispersion is practically zero as shown in FIG. 18. Thus chirp in the laser is not only acceptable, but also desirable in order to reduce the nonlinear effects introduced by the fiber at high levels of launch power. Direct analog modulated ECLs provide enhanced intrinsic distortion performance, but dithering methods (including but not limited to electrical, and mechanical techniques) are typically necessary to counteract the effect of low chirp (stimulated Brillouin scattering—SBS and Rayleigh back-scattering) on analog transmission when high optical power is launched into SMF.
Another method for reducing the nonlinear effects in fibers at 1310 nm is to design the direct analog modulated ECL to emit at a wavelength shifted away from the zero or near-zero dispersion point of the SMF (see FIG. 18). In other words, the emission wavelength of the ECL is intentionally designed and implemented to be in the range of 1320-1350 nm, or after the water peak (that is, >1390 nm).
Another aspect of some embodiments of the present invention relates to optical transmitters incorporating direct analog modulated ECLs, whereby such transmitters are tunable for distortion and/or chirp in the field, and the tuning can be implemented remotely from a central office (CO) or a network operating center (NOC) through network management software. In other words, the analog transmitter's distortion and/or chirp levels and/or optical output power can be tuned or adjusted for an improved or substantially optimized carrier-to-noise ratio (CNR) (and other system parameters) adjusted for specific system links during system equipment installation, or network provisioning, or maintenance. This substantially eliminates the need to procure new sets of transmitters and the associated delays and cost. One embodiment of the tunable analog transmitter includes programming a look-up table into the transmitter memory during the manufacturing of the transmitter. This look-up table contains a “contour map” for desired performance parameters (such as distortion, chirp, wavelength shift, output power vs. TEC temperature). The user can then use software commands to optimize link performance by accessing and selecting from the programmed look-up table.
Another aspect of some embodiments of the present invention relates to wavelength-tunable direct analog modulated ECLs. Embodiments of wavelength-tunable analog ECLs include, but are not limited to, (i) tuning of the FBG (such as using a piezoelectric actuator), for ECLs based on FBG; (ii) grating created and tuned by an RF signal applied to an acousto-optic material, whereby the emission wavelength is tuned and selected by controlling the RF signal, optionally in combination with a movable reflective mirror; (iii) use of an etalon-based tunable filter.
Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.

Claims

1. An analog external cavity laser comprising:

a) a light source having a reflective back facet, a transmissive front facet; and,

b) a partially reflective feedback element forming a laser cavity in cooperation with said reflective back facet; and,

c) one or more temperature controllers maintaining the operating temperature of said laser at least a few degrees Celsius away from a mode hop.

2. An analog external cavity laser as in claim 1 wherein said reflective back facet has a reflectivity exceeding about 75%.

3. An analog external cavity laser as in claim 1 further comprising an anti-reflective coating on said front facet wherein said anti-reflective coating has a reflectivity less than about 0.05%.

4. An analog external cavity laser as in claim 1 wherein said light source is a Fabry-Perot gain element having an active length from about 300 micrometers to about 500 micrometers.

5. An analog external cavity laser as in claim 1 wherein said feedback element is a fiber Bragg grating.

6. An analog external cavity laser as in claim 1 wherein said feedback element has a reflectivity from approximately 5% to approximately 25%.

7. An analog external cavity laser as in claim 1 wherein said light source has a symmetrical far-field beam profile.

8. An analog external cavity laser as in claim 1 wherein said laser cavity has a linewidth enhancement factor at the wavelength of the gain peak of said light source less than about 4.

9. An analog external cavity laser as in claim 1 wherein the ratio of mode spacing to the bandwidth of said feedback element is from approximately 0.5 to approximately 1.3.

10. An analog external cavity laser as in claim 1 wherein the side-lobe suppression of said feedback element exceeds about 15 dB.

11. An analog external cavity laser as in claim 5 wherein said fiber Bragg grating is an apodized grating.

12. An analog external cavity laser as in claim 5 further comprising a microlens on the tip of the fiber containing said fiber Bragg grating.

13. An analog external cavity laser as in claim 1 further comprising an in-line optical isolator.

14. A method of suppressing stimulated Brillouin scattering in an analog external cavity laser comprising modulating the effective light path within the laser cavity.

15. A method of reducing frequency tilt in an analog external cavity laser comprising reducing the dispersive nature of the feedback element of said analog external cavity laser.

16. A method as in claim 15 wherein said method comprises at least one of:

a) using a large bandwidth fiber Bragg grating as said feedback element in the range from approximately 220 picometers to approximately 450 picometers full width at half maximum; and,

b) using a non-apodized fiber Bragg grating as said feedback element; and,

c) using a chirped fiber Bragg grating as said feedback element with appropriate direction to substantially cancel dispersion effects in said grating.

17. An analog external cavity laser as in claim 1 wherein said analog external cavity laser has blue detuning from about 20 nanometers to about 50 nanometers.

18. A method of operating an analog external cavity laser comprising operating said analog external cavity laser under a detuned loading condition such that the slope of the efficiency curve peaks around the threshold current, then decreases as the bias current is increased to reach substantially a minimum value approximately at the desired optical output power, then increases with further increase in said bias current.