Abstract:
A low threshold distributed feedback (DFB) laser is constructed for improved performance at subzero temperatures. A loss grating is employed to enhance the probability that lasing occurs near the short wavelength side of the stopband and to counteract the effect of negative gain tilt that occurs when DFB lasers are positively detuned. A method of making DFB lasers from wafers with improved yield for low temperature side mode suppression ratio (SMSR) is also disclosed.

Description:
FIELD OF THE INVENTION 
     The present invention relates to semiconductor lasers and methods of making and using such devices, and more particularly, to distributed feedback (DFB) lasers that operate satisfactorily at low temperatures. The invention also relates to a method of making DFB lasers from semiconductor wafers and the like. 
     BACKGROUND OF THE INVENTION 
     Distributed feedback (DFB) lasers may be used, for example, as sources of signal radiation for optical fiber communications systems, as optical pumps, and as devices for generating coherent optical pulses. In a conventional DFB device, feedback is provided in the longitudinal direction (the emission direction) by an index grating (a periodic array of materials having different optical indices). In another type of DFB laser, a loss grating is used to provide a periodic variation in loss in the longitudinal direction. 
     There is a need in the art for semiconductor DFB lasers that provide improved performance under conditions, such as low temperature, where they would be positively detuned and suffer degradation in side mode suppression ratio (SMSR, discussed in more detail below). 
     FIG. 1 shows the relationship between output light intensity I and wavelength λ for a typical DFB laser, where: λ m  corresponds to the side mode on the short wavelength side of the stopband; λ o  corresponds to the selected lasing mode; and λ p  corresponds to the side mode on the long wavelength side of the stopband. With reference to FIG. 1, the lasing symmetry L sym  for a particular device may be defined as follows: 
     
       
         (1) Lsym=(λ p −λ o )/(λ p −λ m ).  ( 1 ) 
       
     
     According to equation (1), if the lasing symmetry L sym  is smaller than 0.5, then λ o  is closer to λ p , and if L sym  is greater than 0.5, then λ o  is closer to λ m.    
     The loss function for a DFB laser may be expressed in terms of the coupling coefficient κ, as follows: 
     
       
         (2) κ=κ′+jn, where  ( 2 ) 
       
     
     κ′ represents the real part of the coupling coefficient κ, and jn represents the imaginary part of the coefficient κ. In a pure index grating DFB laser, jn =0, and there is an equal probability of lasing on the long or short wavelength sides of the stopband, as discussed in more detail below. 
     The term “side mode suppression ratio”(or sub-mode suppression ratio) refers to the ratio of main longitudinal mode power to side longitudinal mode power. Some high capacity fiber optic communications systems require a light source that generates a single longitudinal laser mode. A communications system may require a side mode suppression ratio exceeding 30 dB, for example. For a DFB laser to operate in a single longitudinal mode, the side longitudinal modes should be suppressed to relatively insignificant power levels. 
     The side mode suppression ratio (SMSR) changes when the laser is tuned to different operating wavelengths. A DFB laser may exhibit acceptable side mode suppression at certain wavelengths, but unacceptable side mode suppression when tuned to other wavelengths. A small tuning change may cause a 10 dB to 20 dB decrease in the SMSR. This is because the relative net threshold gain required for each mode varies as the laser is tuned. When the main longitudinal mode is centered within the reflection characteristics of the laser, side mode suppression is usually optimized. As the device is tuned away from the optimum position, one of the side longitudinal modes is moved closer to the center of the reflection characteristics. 
     SUMMARY OF THE INVENTION 
     A distributed feedback (DFB) laser having a lasing symmetry L sym &gt;0.5 provides improved performance (high SMSR) under conditions where it would be positively detuned. An example of a condition where a DFB laser would be positively detuned is at low temperatures (for example, −15° C. to −40° C.). Thus, the production yield of DFB devices that perform well at low temperature can be increased by forming them in such a way as to increase the likelihood that the devices have lasing symmetries greater than 0.5. 
     A DFB laser is “positively detuned” when its lasing wavelength (selected mode) is on the long wavelength side of the peak of the material gain spectrum. If the lasing mode is on the long wavelength side, the inherent gain margin is reduced by the relatively large negative gain tilt (dg/dλ) on the long wavelength side. By adding a small amount of loss to the laser grating (that is, by making jn&gt;0), the likelihood that the device will lase on the long-wavelength side of the stop band is reduced. In other words, a DFB laser with a loss grating is more likely to have a lasing symmetry L sym &gt;0.5. If more devices from a wafer are produced with L sym &gt;0.5, then the yield to the low temperature performance requirement is increased. 
     The present invention relates to a method of making low threshold semiconductor DFB lasers for low temperature operation. The method includes the steps of: (1) providing a multi-layer semiconductor structure, such as a wafer, having an active layer, a loss grating, and a spacer layer; and (2) cleaving the multi-layer structure such that opposed facets intersect the loss grating. The cleaving process is such that the precise location at which a particular facet intersects the loss grating cannot be controlled in advance. As discussed in more detail below, the performance characteristics of the laser depend in part on the phase relationships between the randomly determined locations of the facets and the periodic structure of the loss grating. 
     According to a preferred embodiment of the invention, the facets may be covered by highly reflective and anti-reflective coatings, and other structures such as claddings, substrates, electrodes, etc. may also be provided. 
     According to another aspect of the invention, the loss grating may be grown on a semiconductor substrate by a metal organic chemical vapor deposition (MOCVD) process. In a preferred embodiment of the invention, the loss grating has a high As content and therefore is well suited for reliable growth according to an MOCVD process. In another embodiment of the invention, the loss grating may be formed by molecular beam epitaxy. 
     According to another aspect of the invention, lasers are constructed to provide high side mode suppression under conditions where they would be positively detuned. An example of such conditions is where the operating temperature is less than about minus fifteen degrees Celsius (−15° C.). 
     The present invention also relates to a DFB laser for low temperature single mode operation, including: an active layer for producing optical gain; a loss grating for shifting the emission spectrum to the short wavelength side of the stopband; and a spacer layer located between the active layer and the loss grating. According to a preferred embodiment of the invention, the device has a lasing symmetry L sym &gt;0.5. Consequently, the device does not lase on the long wavelength side of the stopband under low temperature conditions. The low operating temperature may be, for example in the range of from −15° to −40° C. 
     According to another aspect of the invention, a DFB laser is constructed to have a high SMSR, to operate at a low threshold, and to exhibit minimal mode hopping, all at low temperatures. 
     According to another aspect of the invention, a device is provided that operates in a single longitudinal mode, with a large gain threshold difference, and that has facet-reflectivity-independent parameters, and low sensitivity to feedback. 
     In a preferred embodiment of the invention, the problems of the prior art are overcome by using a loss grating to shift the distribution of lasing modes, generated by random facet phases, to the short wavelength side of the stopband. The loss grating counteracts the effect of the negative gain tilt that occurs in DFB lasers due to positive detuning at subzero temperatures. According to the present invention, temperature-induced variations of the operational characteristics of a DFB laser are minimized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates the mode intensity distribution for a typical distributed feedback laser. 
     FIG. 2 is a schematic side view of a DFB laser constructed in accordance with a preferred embodiment of the present invention. 
     FIG. 3 is a histogram showing the distribution of lasing symmetries for DFB lasers with index gratings. 
     FIGS. 4 and 5 are histograms showing the distribution of lasing symmetries for DFB lasers with loss gratings, where jn =0.1 and 0.2, respectively. 
     FIG. 6 is a flowchart for a method of making semiconductor lasers according to a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to the drawings, where like reference numerals designate like elements, there is shown in FIG. 2 a distributed feedback (DFB) laser  10  constructed in accordance with a preferred embodiment of the present invention. The device  10  includes a semiconductor substrate  12 , a loss grating  14 , a spacer layer  16 , an active region  18  and a cladding structure  20 . The cladding structure  20  may be formed of plural layers in a manner known in the art. 
     In addition, the device  10  has front and back facets  22 ,  24 . The facets  22 ,  24  may be formed by cleaving the device  10  from a wafer (not shown). An anti-reflective (AR) layer  26  is coated on the front facet  22 . A highly reflective (HR) layer  28  is coated on the back facet  24 . The thicknesses of the two coatings  26 ,  28  correspond to the wavelength of the emitted light  30 . 
     In operation, an electrical current is caused to flow through the active region  18  in a known manner. The current causes the active region  18  to generate light (or gain), such that a signal  30  is emitted through the AR layer  26 . The signal  30  may have a wavelength in the range of from 1.3 μm to 1.6 μm for use in an optical communications system, for example. As discussed in more detail below, the illustrated laser  10  performs satisfactorily, with high side mode suppression, even at low temperatures, for example, in the range of from −15° C. to −40° C. 
     The substrate  12  may be formed of doped InP or another suitable semiconductor material. The loss grating  14  may be formed of InGaAsP, with a high As mole fraction, and is preferably lattice matched to the substrate  12  with a band gap that is 50 nm to 100 nm greater than the targeted lasing wavelength. The loss grating  14  has an alternating, periodic structure in the longitudinal direction of the device  10  (the longitudinal direction is the direction of light emission). The grating period Λ is preferably in the range of from about 2,000 Å to about 2,500 Å. The thickness t of the loss grating  14  may be in the range of from about 200 Å to about 500 Å, for example. 
     The spacer layer  16  is located between the loss grating  14  and a lower portion of the active region  18 . The spacer layer  16  may be formed of InP. In the illustrated embodiment, the thickness  32  of the spacer layer  16  may be in the range of from about 3,000 Å to about 6,000 Å. The present invention should not be limited, however, to the details of the preferred embodiments described herein. 
     The active region  18  may be formed of bulk InGaAsP and/or multiple quantum wells. There may be for example, five to nine quantum wells with a 1% compressive strain. Each quantum well layer may have a width of about 70 Å. 
     Since the device  10  is cleaved from a wafer, the cleaved facets  22 ,  24  intersect the periodic grating  14  at random, arbitrary locations. When a plurality of devices  10  are cleaved from a single wafer (not shown), the performance of each device  10  depends on the randomly determined locations of the facets  22 ,  24  relative to the grating  14 . The loss grating  14  is formed across the entire wafer before the cleaving operation. Consequently, the lasers  10  have facets  22 ,  24  intersecting the periodic loss grating  14  at different phases Ø. Some devices  10  will perform differently than others, depending on the phases Ø of the loss grating  14  at the points of intersection with the facets  22 ,  24 . 
     The relationship between the facet reflectivity R on either end of the device  10  and the respective facet phase Ø is as follows: 
     
       
         R=e iØ .  (3) 
       
     
     By employing equation (3), the expected lasing symmetry L sym  for a cleaved laser  10  can be calculated for each possible combination of facet phases Ø. FIGS. 3-5 show histograms of L sym  values for distributions calculated by varying the phases Ø at the ends of different devices in steps of 10°, from 0° to 350°, for a total of 1296 facet phase combinations for each distribution. For each histogram, the lasing symmetry distribution is characterized as a function of μ and σ, where μ represents the calculated numerical median of the distribution and σ represents the standard deviation, on the assumption that it would be normal. 
     FIG. 3 shows the distribution of lasing symmetries for DFB lasers with pure index gratings (κ is real). For κL=1.5, the calculated median is μ 1 =0.4998, at a calculated standard dcviation σ 1 =0.1573. FIGS. 4 and 5, in contrast, show the distribution of lasing symmetries for DFB lasers  10  with loss gratings  14 , where jn=0.1 and 0.2, respectively. In the FIGS. 4 and 5 distributions, the complex part jn of the coupling cocfficient κ is nonzero because the bandgap wavelength of the grating  14  is greater than the wavelength of the emitted radiation, and absorption occurs. 
     As shown in FIG. 4, the lasing symmetry distribution is shifted, and the calculated median is higher than the median for a pure index grating DFB laser because λ o  is closer to λ m . The calculated median for FIG. 4, where jn=0.1, is μ 2 =0.5859, at a standard deviation σ 2 =0.1524. 
     With respect to FIG. 5, where the loss is increased relative to FIG. 4, the combination of facet phases yielding a lasing mode on the long wavelength side of the stopband is smaller than that calculated for FIG.  4 . For FIG. 5, the calculated median is μ 3 =0.6435, considerably higher than the FIG. 4 value, at a standard deviation σ 3 =0.1322. 
     Thus, the loss grating  14  may be used to increase the probability that a laser  10  cleaved from a wafer (not illustrated) operates on the short wavelength side of the stopband. As demonstrated in FIGS. 4 and 5, the percentage of devices that lase on the long-wavelength side is smaller, and consequently the yield for high SMSR at low temperature increases. Because of the more advantageous distribution for low temperature performance, an increased number of low temperature devices can be cleaved from a single wafer. 
     Referring now to FIG. 6, a wafer (not illustrated) may be constructed  100  with layers corresponding to the substrate  12 , loss grating  14 , spacer  16 , active layer  18 , and cladding structure  20 . The loss grating layer advantageously may be formed by an MOCVD process. Each layer of the wafer (including the loss grating layer) may extend with a constant thickness across essentially the full extent of the wafer. The wafer may then be cleaved  102  into a large number of devices. The cleaving process  102  forms the facets  22 ,  24  at different phases Ø of the loss grating  14 . The facets  22 ,  24  are then coated  104  with the desired coatings  26 ,  28 , and the finished lasers  10  are then tested  106  for performance characteristics. The lasers  10  that will perform well at low temperatures may be selected or identified (by, for example, analyzing the DFB mode spectroscopy) for appropriate low temperature applications. A spectrum analyzer may thus be used to select only the short-wavelength devices cleaved from the wafer. 
     The above description illustrates preferred embodiments which achieve the objects, features and advantages of the present invention. It is not intended that the present invention be limited to the illustrated embodiments. Any modification of the present invention that comes within the spirit and scope of the following claims should be considered part of the present invention.