CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119 of U.S. Provisional Application Serial No. 61/773316 filed on March 06, 2013 and Application Serial No. 13/837270 filed on March 15, 2013 the contents of which are relied upon and incorporated herein by reference in their entirety.

FIELD

[0001] This disclosure is related to lasers, and in particular relates to a high-power, low-loss gradient-index separate-confinement heterostructure (GRINSCH) laser.

BACKGROUND ART

[0002] Semiconductor lasers are widely used as pumping sources for solid-state lasers, fiber lasers and light sources for direct-diode systems. These laser systems are rapidly deployed into material processing, medical and electronics markets.

[0003] Semiconductor lasers are generally formed from layers of doped semiconductor materials that define an active layer and surrounding cladding layers. Conventional semiconductor lasers have an abrupt transition between the active and cladding layers, which defines an abrupt change in the ba nd gap between the materials. One newer type of semiconductor laser is called a high-power, low-loss gradient-index separate-confinement heterostructure or GRINSCH laser. GRINSCH lasers have layers where the doping changes gradually so that the band gap changes gradually. This generally results in greater efficiency as compared to conventional semiconductor lasers.

[0004] Yet, there is an ongoing need for GRINSCH lasers that have reduced waveguide loss and reduced carrier leakage. SUMMARY

[0005] An aspect of the disclosure is a GRINSCH laser having an asymmetric configuration wherein the optical confinement is weighted more to the n-doped multilayer region than to the p-doped multilayer region. The GRINSCH laser can emit laser light at a wavelength λ = 976 nm over a broad area with a beam power of 11.4 W at a 12 A bias current and 20 °C heat sink temperature. Embodiments include Fabry-Perot (FP) and distributed Bragg reflector (DBR) GRINSCH laser configurations.

[0006] It is to be understood that both the foregoing general description and the following Detailed Description represent embodiments of the disclosure, and are intended to provide an overview or framework for understanding the nature and character of the disclosure as it is claimed. The accompanying drawings are included to provide a further understanding of the disclosure, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the disclosure and together with the description serve to explain the principles and operations of the disclosure.

[0007] Additional features and advantages of the disclosure are set forth in the detailed description that follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the disclosure as described herein, including the detailed description that follows, the claims, and the appended drawings.

[0008] The claims as set forth below are incorporated into and constitute part of the Detailed Description set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1A is a schematic diagram of an example Fabry-Perot (FP) GRINSCH laser as disclosed herein;

[0010] FIG. IB is a schematic diagram of an example distributed Bragg reflector (DBR) GRINSCH laser as disclosed herein; [0011] FIGS. 2A and 2B a re schematic diagrams of the layered structure of exa mple prior a rt GRI NSCH lasers;

[0012] FIGS. 2C and 2D are schematic diagra ms of a n exam ple of the layered structure of a n example DBR GRI NSCH laser;

[0013] FIG. 3A is a plot of the refractive index n versus the position x (μιη) for the prior a rt GRINSCH laser as set forth in Table 2;

[0014] FIG. 3B is a plot of the adj usted refractive index n' a nd the intensity distribution of the laser beam as a function of position x (μιη) for the prior art GRI NSCH laser as set forth in Ta ble 2;

[0015] FIG. 4A is a plot of the refractive index n versus the position x (μιη) for the FP GRINSCH laser as set forth in Table 3;

[0016] FIG. 4B is a plot of the adj usted refractive index n' a nd the intensity distribution of the laser bea m as a function of position x ( μιη) for the FP GRI NSCH laser as set forth in Table 3;

[0017] FIG. 5A is a plot of the voltage V (volts V) (solid line) and laser output power P (Watts W) (dashed line) versus the current i (Amperes A) for the FP GRI NSCH laser of Table 3;

[0018] FIG. 5B is a plot of the heat H (W) (solid line) and wall plug efficiency WPE (unitless) (dashed line) versus the cu rrent i (A) for the FP GRI NSCH laser of Table 3;

[0019] FIG. 5C plots the laser intensity I (arbitra ry units) versus the lasing wavelength λ (nm) for different bias currents (0.5A to 12A at 0.5A per step) for the FP GRI NSCH laser of Table 3;

[0020] FIG. 5D is a plot of the lasing wavelength λ (nm) (solid line) and the lasing spectral width B (nm) (dashed line) as a function of current i (A) for the FP GRI NSCH laser of Table 3;

[0021] FIG. 6A is a plot of the refractive index n versus the position x (μιη) for the DRB GRINSCH laser as set forth in Tables 4A and 4B; [0022] FIG. 6B is a plot of the adj usted refractive index n' and the intensity distribution of the laser beam as a function of position x (μιη) for the DBR GRI NSCH laser as set forth in Tables 4A and 4B;

[0023] FIG. 7A is a plot of the voltage V (volts V) (solid line) and laser output power P (Watts W) (dashed line) versus the current i (Amperes A) for the DRB GRI NSCH laser of Tables 4A and 4B;

[0024] FIG. 7B is a plot of the heat H (W) (solid line) and wall plug efficiency WPE (unitless) (dashed line) versus the cu rrent i (A) for the DRB GRI NSCH laser of Tables 4A a nd 4B;

[0025] FIG. 7C plots the laser intensity I (arbitra ry units) versus the lasing wavelength λ (nm) for different values of bias current [0.5-12A at 0.5 A per step] for the DRB GRI NSCH laser of Tables 4A a nd 4B;

[0026] FIG. 7D is a plot of the lasing wavelength λ (nm) (solid line) and the lasing spectral width B (nm) (dashed line) as a function of current i (A) for the DRB GRI NSCH laser of Tables 4A and 4B; and

[0027] FIGS. 8A through 8F and FIGS. 9A through 9F are plots associated with a sensitivity studies of the design parameters for an FP or a DBR GRI NSCH laser having select design properties.

[0028] Additional features and advantages of the disclosure are set forth in the Detailed Description that fol lows and will be apparent to those skilled in the art from the description or recognized by practicing the disclosure as described herein, together with the claims and appended drawings.

[0029] Cartesian coordinates are shown in certain of the Figures for the sake of reference and are not intended as limiting with respect to direction or orientation. DETAILED DESCRIPTION

[0030] FIGS. 1A and IB are schematic diagrams of different basic configurations of an example GRINSCH laser 10 as disclosed herein. The GRINSCH laser 10 of FIG. 1A has a Fabry- Perot (FP) configuration, while the GRINSCH laser 10 of FIG. IB has a distributed Bragg reflector (DBR) configuration. GRINSCH laser 10 has a laser wavelength λ that can be in the range from 900 to 999 nm, e.g., at a nominal wavelength λ = 976 nm. The laser wavelength λ is thus usually denoted as "9xx."

[0031] The FP GRINSCH laser 10 of FIG. 1A includes in order from bottom to top: a n- contact layer 12N, an n-cladding layer 13NC, an n-sub-cladding layer 13NSC an n-doped multilayer section 20n (also called the "n-GRINSCH"), a lasing layer 14, a p-doped multilayer section 20p (also called the "p-GRINSCH"), a p-sub-cladding layer 13PSC, a p-cladding layer 13P, and a p-contact layer 12P. GRINSCH laser 10 also includes front and rear facets 22F and 22R that define a gain section GS that extends between the facets. GRISNCH laser 10 is configured to emit laser light 40 having the aforementioned wavelength λ.

[0032] With reference to FIG. IB, DBR GRINSCH laser 10 is similar to the FP GRINSCH laser of FIG. 1A and includes a DBR section ("DBR") and an optional phase section φ.

[0033] FIG. 2A is a schematic diagram of the layered structure of an example GRINSCH laser 10 that is constituted by layers LI through L20. Table 1 below describes the general layers LI through L20 of a prior art GRINSCH laser, starting at the top of the Table with the topmost layer L20 and proceeding downward to the bottom-most layer LI at the bottom of the Table 1.

[0034]

[0035] Layers L3 through L6 constitute n-doped multilayer section 20n and layers L10 through L17 constitute p-doped multilayer section 20p. Layer LI is the n metal contact 12N and layer 18 is the p metal contact 12P. The quantum well L8 corresponds to laser layer 14 in FIGS. 1A and IB. The quantum well L8 is sandwiched by barrier layers L7 and L9. Prior art symmetrical GRINSCH laser example

[0036] Table 2 below sets forth the specific layer parameters associated with an example prior art configuration of GRINSCH laser 10. FIG. 2B is representative of the general layered structure of the FP GRINSCH laser 10 of Table 2. The notation (X->Y) in the "Doping" column indicates the doping gradient over the particular layer.

[0037] Layers L6 and L14 have photoluminescent center wavelengths A _L of 700 nm, whi le lasing layer L9 has a photoluminescent center wavelength of 960 nm.

[0038] FIG. 3A is a plot of the layer refractive index n versus position X (in microns or μιη) showing the relatively symmetric refractive index profile of the GRINSCH laser of Table 2B a bout the lasing layer L9, which is centered at X = 0. FIG. 3B is a plot of the (adjusted) refractive index n' = n-3 and the norma lized intensity I as a function of the X coordinate. The GRI NSCH laser of Ta ble 2 has a (976L28Y, Y=25) configuration. The intensity plot indicates that the light 40 emitted by the GRI NSCH laser travels in substantially the same amounts in the n-section 20p and the n-section 20n. However, for a given carrier concentration, the optical loss due to a hole is a bout three times that due to an e lectron, so that this distribution of light 40 is not optima l.

[0039] Accordingly, it would be beneficia l to have more of the light 40 travel in the N-doped multilayer section 20p, and in particular, travel mostly in the low doped n-GRI NSCH region) rather than in the p-doped m ultilayer section 20p. Moreover, it would be beneficial to reduce the carrier lea kage at high temperature by increasing the barrier height of the quantum well that constitutes lasing layer L9.

FP GRINSCH Laser

[0040] Ta ble 3 sets forth exam ple pa ra meters for a n embodiment of FP GRI NSCH laser 10 of FIG. 1A as disclosed herein. FIG. 2C is a schematic diagram similar to FIG. 2A that shows the layered structure of the FP GRI NSCH laser 10 of Ta ble 3. The pa ra meter T associated with layer L4 is a thickness parameter and can have the value of 0.5, 1, 1.5 or 2 μιη.

[0041]

[0042] Layers L6 and L14 have photoluminescent center wavelengths A _L of 700 nm, wh lasing layer L9 has a photoluminescent center wavelength of 960 nm. [0043] FIGS. 4A and 4B are the same type of plots as FIGS. 3A and 3B except that they are for the FP GRINSCH laser 10 as set forth in Table 3. The step-wise profile of the refractive index plots of FIGS. 3A and 3B is a quantized approximation used in the modeling calculations, and in actuality the curve is generally continuous.

[0044] The configuration of FP GRINSCH laser 10 is asymmetrical about lasing layer L9 and has a reduced doping structure. Asymmetrical GRINSCH laser 10 of Table 3 is modified from the symmetrical GRINSCH laser of Table 2 by half the p-GRINSCH and triple the n-GRINSCH, with the result that more light 40 is confined to N-section 20n (i.e., mostly in the low doped n-GRINSCH region) than in p-section 20p. Moreover, the amount of doping near the GRINSCH region is reduced by a factor of three. Both of these modifications substantially reduce the waveguide loss and result in improved laser efficiency as compared to the GRINSCH laser of Table 2.

[0045] An example of the FP GRINSCH laser 10 as disclosed herein emits laser light 40 at a wavelength λ = 976 nm over a broad area with a beam power of 11.4 W at a 12 A bias current and 20 °C heat sink temperature.

[0046] FIG. 5A is a plot of the voltage V (volts V) (solid line) and laser output power P (Watts W) (dashed line) versus the bias current i (Amperes A) for the FP GRINSCH laser of Table 3 with T=0.5.

[0047] FIG. 5B is a plot of the heat H (W) (solid line) and wall plug efficiency WPE (unitless) (dashed line) versus the bias current i (A) for the FP GRINSCH laser of Table 3 with T=0.5.

[0048] FIG. 5C plots the laser intensity I (arbitrary units) versus the lasing wavelength λ (nm) for different values of the bias current i for the FP GRINSCH laser of Table 3 with T=0.5 .

[0049] FIG. 5D is a plot of the lasing wavelength λ (nm) (solid line) and the lasing spectral width B (nm) (dashed line) as a function of the bias current i (A) for the FP GRINSCH laser of Table 3 with T=0.5.

DBR GRINSCH laser

[0050] Tables 4A and 4B set forth example parameters for an embodiment of DBR GRI NSCH laser 10 of FIG. IB. FIG. 2D is representative of the layered structure of the FP GRINSCH laser 10 of Tables 4A and 4B. DBR GRI NSCH laser 10 is formed using two growth steps, with the first growth step presented in Table 4A a nd the second growth step presented in Table 4B.

[0051] The structure of DBR GRINSCH laser 10 associated with the re-growth or second growth step is represented in Table 4B, below, and shown in FIG.2C. The typical values for Tl and T2 are 315 and 300 nm, respectively.

[0052]

[0053] FIGS.5A and 5B are the same type of plots as FIGS.4A and 4B except that they are for the DBR GRINSCH laser 10 as set forth in Tables 4A and 4B. The DBR GRINSCH laser 10 as described in Tables 4A and 4B define an asymmetrical GRINSCH configuration with reduced doping and high barrier structure. The aluminum content in the barrier section (layer L10) in p- section 20p is increased from 8% to 13% (relative to the prior art configuration of Table 2) to prevent electron overflow from the quantum well (layer L9) at high temperature. This improves the linearity of the light-current curve at high current, which allows the DBR GRINSCH laser 10 to emit a high-power laser beam 40.

[0054] An example of the broad area DBR GRINSCH laser 10 as disclosed herein emits laser light 40 at a wavelength λ = 976 with a power of 10 W at a 12 A bias current and a heat sink temperature of 20 °C. [0055] FIG. 7A is a plot of the voltage V (volts V) (solid line) and laser output power P (Watts W) (dashed line) versus the bias current i (Amperes A) for the DRB GRINSCH laser of Tables 4A and 4B.

[0056] FIG. 7B is a plot of the heat H (W) (solid line) and wall plug efficiency WPE (unitless) (dashed line) versus the bias current i (A) for the DRB GRINSCH laser of Tables 4A and 4B.

[0057] FIG. 7C plots the laser intensity I (arbitrary units) versus the lasing wavelength λ (nm) for different values of the bias current i for the DRB GRINSCH laser of Tables 4A and 4B.

[0058] FIG. 7D is a plot of the lasing wavelength λ (nm) (solid line) and the lasing width B (nm) (dashed line) as a function of the bias current i (A) for the DRB GRINSCH laser of Tables 4A and 4B.

Example Performance Conditions and Design Parameters

[0059] An example GRINSCH laser 10 preferably satisfies a number of performance conditions. A low threshold current i requires quantum well optical confinement factor (Q.WCF) to be greater than 1% for a laser cavity that is longer than 4 mm. In addition, high coupling efficiency to an optical fiber requires vertical far field angles VFFA (measured at full-width half- maximum) to be less than 30 degrees.

[0060] Furthermore, a high slope efficiency SE (i.e., low optical loss) requires an optical confinement factor in the undoped or low-doped region such as in the GRINSCH to be greater than 60%. High-temperature performance requires the aluminum mole fraction of the Q.W barrier layers to be 8% or higher.

[0061] FIGS. 8A through 8F a nd FIGS. 9A through 9F are plots associated with a sensitivity studies of the design parameters for an FP or DBR GRINSCH laser 10 having the above properties. These plots define the design space for example GRINSCH lasers 10.

[0062] FIGS. 8A and 9A plot the quantum well optical confinement factor QWCF (%) (left vertical axis; diamonds) and the vertical far-field angle VFFA (degrees)(right vertical axis;

squares) vs. the sub-cladding layer thickness (microns) of the Alo.21Gao.79As and AI0.23GA0.77AS layers, respectively. FIGS. 8B and 9B plot the quantum well optical confinement factor Q.WCF (%) (left vertica l axis; diamonds) and the vertical far-field angle VFFA (degrees)(right vertical axis; squares) vs. the thickness (nm) of the n-GRINSCH layer 20n.

[0063] FIGS. 8C and 9C plot the quantum well optical confinement factor QWCF (%) (left vertical axis; diamonds) and the vertical far-field angle VFFA (degrees)(right vertical axis;

squares) vs. the aluminum mole fraction of sub-cladding layers (see, e.g., layers L5B in Table 3 and 4A).

[0064] FIGS. 8D and 9D plot the optical confinement factor (%) vs. the thickness (nm) of the Alo.21Gao.79As layer and Alo.23Gao.77As layers, respectively, for the p-cladding layer 20p

(diamonds), the n-cladding layer 20n (squares) and the GRINSCH region (triangles).

[0065] FIGS. 8E and 9E plot the optica l confinement factor (%) vs. the n-GRINSCH thickness (nm), along with a dotted vertical reference line indicated the example experimentally demonstrated, for the p-cladding layer (diamonds), n-cladding layer (squares) and the GRINSCH region (triangles). FIGS. 8F and 9F plot the optical confinement factor (%) vs. the Aluminum mole fraction, for the p-cladding layer (diamonds), n-cladding layer (squares) and the GRINSCH region (triangles).

[0066] The dotted line in the plots represents example design values that satisfy the above- identified performance conditions. I n one example, the given design parameter can vary by +/- 10% from the value indicated by the vertical dotted line, while in another example, the given design parameter can vary by +/- 5%.

[0067] Although the embodiments herein have been described with reference to particular aspects and features, it is to be understood that these embodiments are merely illustrative of desired principles and a pplications. It is therefore to be understood that numerous

modifications may be made to the illustrative em bodiments and that other arrangements may be devised without departing from the spirit and scope of the appended claims.