CURRENT BIASED DUAL DBR GRATING SEMICONDUCTOR LASER

Abstract: A laser heterostructure (10) having an active layer, a lateral waveguide terminating in an output aperture (11), and a gain section (17) with a current drive electrode. A rear surface distributed Bragg grating (12) with a tuning current electrode (19) is formed on a surface of said laser heterostructure. The laser also includes a front surface distributed Bragg grating (14) with a tuning current electrode (16) on a surface of the laser heterostructure. The front surface distributed Bragg grating is closer to the output aperture than the rear surface distributed Bragg grating. There is a space (15) between the rear surface distributed Bragg grating and the front surface distributed Bragg grating. A current drive electrode (18) is formed on the space. Operation is best when the front surface distributed Bragg grating has adequate reflectivity at the Bragg wavelength with minimal scattering loss at other wavelengths, particularly at the wavelength of the rear surface Bragg grating.

Description: CURRENT BIASED DUAL DBR GRATING SEMICONDUCTOR LASER TECHNICAL FIELD The field of the invention is semi conductor lasers. BACKGROUND ART Multiwavelength optical sources are important components in applications such as wavelength division multiplexing, optical remote sensing, and optical data processing. Multiple wavelengths are commonly achieved from a single output by integrating the output from multiple, discrete lasers. This can lead to large and complex chip design, however. Other approaches, such as cascaded strongly gain-coupled DFB (distributed feedback) lasers, rely on the reflectivity comb from integrated multi wavelength feedback mechanisms for their operation. Such integrated design of the multi wavelength feedback makes it difficult to select and tune a wavelength while leaving other wavelength(s) unaffected. Thus, there is a need for an improved semiconductor laser capable of dual wavelength operation. It is an object of the invention to provide such a laser. DISCLOSURE OF THE INVENTION These needs are met by the present a laser heterostructure having an active layer, a lateral waveguide terminating in an output aperture, and a gain section with a current drive electrode. A rear surface distributed Bragg grating with a tuning current electrode is formed on a surface of the laser heterostructure. The laser also includes a front surface distributed Bragg grating with a tuning current electrode on a surface of the laser heterostructure. The front surface distributed Bragg grating is closer to the output aperture than the rear surface distributed Bragg grating, There is a space between the rear surface distributed Bragg grating and the front surface distributed Bragg grating. A current drive electrode is formed on the space. Operation is best when the front surface distributed Bragg grating has adequate reflectivity at the Bragg wavelength with minimal 5 scattering loss at other wavelengths, particularly at the wavelength of the rear surface Bragg grating. In the present laser, dual- wavelength operation is easily achieved by biasing the gain section. A relatively low coupling coefficient, K, in the front grating reduces the added cavity loss for the back grating mode. Therefore, the back grating mode reaches l o threshold easily. The space section lowers the current induced thermal interaction between the two uniform grating sections, significantly reducing the inadvertent wavelength drift. As a result, a tunable mode pair separations (Δλ) as small as 0.3 nm and as large as 6.9 run can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS 15 FIG. 1 is a schematic diagram of a preferred RW-DBR laser heterostructure of the invention; FIG. 2 illustrates threshold currents for three prototype devices of the invention measured at different temperatures ranging from 20°C to 60°C; FIG. 3 illustrates longitudinal mode spectra of a prototype device measured 20 at different tuning currents from O mA to about 35 mA with the laser current fixed at 50 MA at 60°C; and FIG.4 shows peak wavelengths of the prototype laser measured in FIG. 3. 5 MODES FOR CARRYING OUT THE INVENTION Referring now to FIG. 1, a preferred ridge waveguide-distributed Bragg reflector (RW - DBR) laser heterostructure 10 of the invention is shown. The laser heterostructure emits from an output aperture 11. A rear surface distributed Bragg grating 12 is formed on a surface of the laser heterostructure. The laser also includes a front surface distributed Bragg grating 14 on a surface of said laser heterostructure. The front surface distributed Bragg grating 14 is closer to the output aperture 11 than the rear surface distributed Bragg grating 12. There is a space 15 between the rear surface distributed Bragg grating 12 and the front surface distributed Bragg grating 14. A tuning electrode 16 is formed on the front surface distributed Bragg grating 14, and the region between the front distributed Bragg grating and the output aperture 11 forms a gain section 17. A current drive electrode 18 is formed on the space 15, and a tuning electrode 19 on the rear surface distributed Bragg grating. An additional current drive electrode 20 is formed on the gain section 17. Operation is best when the front surface distributed Bragg reflector 14 has an optimal design such that adequate reflectivity at the Bragg wavelength is obtained with minimal scattering loss at other wavelengths, particularly that of the rear surface distributed Bragg grating 12. The coupling coefficient of the rear surface distributed Bragg grating 12 is less critical and may be higher than that of the front surface distributed Bragg grating 14. Prototypes have been formed and tested. Their operation illustrates some advantages of the invention, but the particular prototype parameters, i.e. grating dimensions, separation lengths, etc., do not limit the broader aspects of the overall structure which forms part of the invention as shown in FIG. 1. The prototypes and their operation will now be discussed. In prototypes which were tested, uniform front and back gratings with an identical period were used, and current injection into the tuning electrode 16 defined the wavelength separation between the mode pairs. The laser operates in a single-wavelength mode with no current applied to the tuning electrode 16, and dual- wavelength operation is achieved when current is applied to the tuning electrode 16. The prototype InGaAs-GaAs asymmetric cladding RW-DBR lasers operate in a dual-wavelength mode with tunable mode pair separations as small as 0.3 nm and as large as 6.9 nm. Epitaxial layers for the asymmetric cladding separate confinement heterostructure (SCH) prototypes were grown by atmospheric pressure metalorganic chemical vapor deposition (MOCVD) in a vertical reactor on a (100) GaAs:n + substrate. After the growth, silicon dioxide was deposited by plasma-enhanced chemical vapor deposition. Grating periods were defined by electron beam lithography. The gratings were transferred into the silicon dioxide using Freon 23 reactive ion etching ;(RIE). Plasma-Therm SLR inductively-coupled plasma (ICP) RIE was used to etch the gratings into the epitaxial layers with SiCl 4 . A 4 μm lateral ridge was defined using standard photolithography and etched in a sulfuric acid solution. A liftoff metallization was performed to provide separate contacts for the laser and the tuning elements. No coatings, are applied to the facets. The prototype DBRs utilize uniform gratings with the same Bragg period, but gratings having different periods will also work within the above described limitations of reflectivity and scattering losses. The metal liftoff was aligned over the gratings to provide electrical isolation for the common gain section and the two DBR sections. The two DBR sections are physically separated in order to reduce the inadvertent heating mentioned. Three different device dimensions were investigated. Table I is a summary of physical dimensions of these devices. The ridge heights and ridge widths for all three devices are 0.15 μm and 3.5 μm, respectively. All prototype DBRs had a uniform grating with a period of 166.0 nm. TABLE I Device (a) (b) (c) •t^gain 500 525 550 Lfront DBR 100 75 50 -'-'space 100 75 50 Lback DBR 100 100 100 Prototype testing was performed continuous wave (CW) with a heat sink temperature maintained by a thermoelectric (TE) cooler. Only the gain section is biased to achieve lasing. FIG. 2 shows threshold currents for the three different devices in Table, I measured at different temperatures. At 20 °C, the threshold currents for devices (a)».(b), and (c) are 12 mA, 28 mA, and 32 mA, respectively, and the uniform grating period of 166.0 nm results in a single nominal lasing wavelength (λ front of ~1077.4 nm: 1 As ; expected, device (a), with the shorter gain section and the longer front DBR section (i.e. stronger reflectivity), exhibits the lowest threshold current for all temperatures. FIG. 2 also shows that the threshold currents decrease for all three devices as the TE cooler temperature is increased from 20°C to 60°C. Threshold currents for devices (a), (b), and (c) are as low as 7.1 mA, 18 mA, and 18.5 mA, respectively. This trend can be accounted for by the Bragg condition being red-shifted with respect to the gain peak. As the temperature of the device increases, the gain peak tunes toward the Bragg condition, leading to a lower threshold current. As expected, no lasing is observed when only the tuning pad is biased. Dual- wavelength operation occurs in devices (b) and (c) when the lasers are biased high enough to support two lasing modes. Both devices exhibit similar trends in their performance. No additional current is required for dual- wavelength operation. The coupling coefficient, K, of the front DBR is estimated to be ~ 80 cm "1 . This lower value of K is believed to be an important feature that contributes to the dual- wavelength operation of these devices. For these devices with deeply etched surface gratings, the higher threshold condition for the second lasing mode for the back DBR (λ back ) results from the added cavity loss caused by scattering in the front DBR. By reducing the loss caused by the front grating section, λ back reaches threshold more easily. While device (a) exhibits the lowest threshold current, it does not exhibit dual-wavelength operation. Because the added loss for λ back in device (a) with the longest front grating (L f ) and the spacing (L s ) sections, λ back does not reach threshold. Therefore, a low KL product for the front grating is an important factor determining the threshold condition of λ bac . However, K for the front grating still needs to be sufficiently high in order for the front grating to act as a good wavelength-selective reflector. Minimizing cavity loss for the back grating section should be carefully balanced with sufficient reflection for λ ack ' Once dual-wavelength operation is achieved by biasing the gain section, injection of current into the front DBR section results in wavelength tuning. FIG. θ shows the longitudinal mode spectra of device (c) measured at different tuning currents from 0 mA to 35 mA with the laser current fixed at 50 mA at 60°C. As noted earlier, the gain peak of the material overlaps better with the Bragg condition at elevated operating temperatures, facilitating dual-wavelength operation and therefore, tuning. Because of thermal coupling between the gain section and the front DBR, the wavelength of λ front is slightly longer than λ back . FIG. 4 shows the peak wavelengths the RW-DBR laser presented in FIG. 3. Closed circles represent the front DBR mode ont an open circles represent the back DBR mode (λ ack ). As the tuning current increases, both modes shift towards longer wavelengths, with the tunable wavelength shifting further. When the tuning current is increased from 0 mA to 35 mA, λ &on t increases by 7.4 nm, tuning from 1081.6 nm to 1089.0 nm. Therefore, the RW-DBR laser exhibits a tunable mode pair separation (Δλ) as small as 0.3 nm and as large as 6.9 nm. At higher currents λ front jumps a lower wavelength that is more favorable for lasing. Device (b) also exhibits similar trends. FIG.4 also illustrates the effect of inserting a 50-μm spacing section between the to reduce the inadvertent drifting of λ ack . When the tuning current is increased from 0 mA to 40 mA, the wavelength of λ ack increases by only 0.9 nm, from 1081.3 nm to 1082.2 nm (0.025 nm/mA). For device (b), which has a longer spacing section (75 μm), the drift in λ back is further reduced to ~ 0.015 nm/mA. Thus, by simply biasing the gain section, a laser of the invention operates in dual- wavelength. Lower coupling coefficient, K, in the front grating reduces the added cavity loss for the back DBR mode λ bac and therefore, λ back reaches threshold more easily. Also, the addition of a spacing section reduces the current induced thermal interaction between the two uniform grating sections, significantly reducing the inadvertent wavelength drift. As a result, a tunable mode pair separations (Δλ) as small as 0.3 nm and as large as 6.9 nm can be achieved. ■ ' > While various embodiments of the present invention have been shown and described, it should be understood that other modifications, substitutions and alternatives are apparent to one of ordinary skill in the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the invention, which should be determined from the appended claims. Various features of the invention are set forth in the appended claims.

Claims: WHAT IS CLAIMED IS: 1. A semiconductor laser, the laser comprising: a laser heterostructure (10) having an active layer, a lateral waveguide terminating in an output aperture (11), and a gain section (17); and a rear surface distributed Bragg grating (12) on a surface of said laser heterostructure; a tuning electrode (19) formed on said rear surface distributed Bragg grating; a front surface distributed Bragg grating (14) on a surface of said laser heterostructure and closer to said output aperture than said rear surface distributed Bragg grating; a tuning electrode (16) formed on said front surface distributed Bragg grating; a space (15) between said rear surface distributed Bragg grating and said front surface distributed Bragg grating; a current drive (20) electrode formed on said gain section; a current drive (18) electrode formed on said space. 2. The laser according to claim 1 , wherein said front surface distributed Bragg grating has a period simultaneously providing adequate reflectivity at a Bragg wavelength of said front surface distributed Bragg grating and minimizing scattering loss at a wavelength of said rear surface distributed Bragg grating. 3. The laser according to claim 2, wherein said rear surface distributed Bragg grating and said front surface distributed Bragg grating have a common Bragg period. 4. The laser according to claim 2, wherein said laser heterostructure comprises InGaAs-GaAs in a ridge waveguide distributed Bragg reflector formation. 5. The laser according to claim 2, wherein the coupling coefficient of said front distributed Bragg gratings is approximately 80 cm - " 1. 6. The laser according to claim 1 , wherein a coupling coefficient of a front one of said pair of surface distributed Bragg grating is sufficient high to provide wavelength selective reflectivity and sufficiently low to reduce cavity loss for a rear one of said pair of surface distributed Bragg gratings. :

Inventors: COLEMAN JAMES J, ROH S DAVID, COLEMAN, JAMES, J., ROH, S., DAVID
Applicants: TRUSTEES OF THE UNIVERSITY OF, THE BOARD OF TRUSTEES OF THE UNIVERSITY OF ILLINOIS
Application year: 2001
Main classification: H01S5
IPC codes: H01S3, H01S5