M. Achtenhagen, N.V. Amarasinghe, and G.A. Evans*
1155 E. Collins Blvd., Suite 200
Richardson, Texas 75081
* Department of Electrical Engineering
Southern Methodist University
Dallas, Texas 75275
This letter reports on distributed Bragg reflector lasers emitting in excess of 700 mW in a single-spatial and single-spectral mode at 1064 nm. The threshold current of these devices is ~30 mA, an L-I slope of 0.74 W/A and a side mode suppression ratio greater than 30 dB. The current and thermal tuning is 0.016 Å/mA and 0.7 Å/ºC.
High-power single-frequency laser diodes emitting around 1064 nm are of interest for a variety of applications including direct replacement of Nd:YAG lasers, spectroscopy, gas-sensing or THz generation [1–4]. These first-order distributed Bragg reflector (DBR) ridge waveguide lasers, fabricated with holographic lithography and without regrowth, are expected to have high reliability and low cost in high volume production.
The DBR lasers were processed from an MBE grown structure consisting of an n-cladding layer, an InGaAs single quantum well (SQW), which is centred in a 300 nm-thick AlGaAs graded region, a p-cladding layer and a highly-doped GaAs cap layer. A detailed structure and basic processing steps for such ridge-type lasers have been published in . The DBR lasers consist of a 400 mm-long DBR section that provides selective feedback using a first-order grating fabricated by holographic lithography with a grating period of ΛB = 1608 Å. The grating coupling coefficient is 50 cm-1 and is controlled by the etch depth. The gain section has a length of 2000 mm and the width of the ridge is about 3 micrometers. The optical cavity is formed by the rear DBR mirror and a cleaved and coated front facet.
Fig. 1 Front facet power (L) and junction voltage (v) against drive current (I) of DBR laser operating CW at room temperature. Inset: Effective DBR reflectivity against wavelength; points indicate different stable modes.
Following device fabrication, the wafers are thinned to approximately 100 mm and metallised. After bar-cleaving and passivation of the front and rear facet an antireflection coating of around 5% is applied to the front facet. Single devices are separated using conventional dicing techniques and are mounted junction-side down on c-mounts. All measurements are made under continuous-wave (CW) conditions and at room temperature.
Fig. 2 DBR laser emission spectra from 200 to 600 mW in steps of 100 mW
Fig. 1 shows optical power from the front facet and voltage against injection current for a typical DBR laser device. The device has a threshold current of 30 mA and the slope of the L-I curve above threshold is 0.74 W/A. At a maximum applied injection current of 1.0 A the emitted power exceeds 700 mW. The series resistance Rs = 1.0 Ω. Although the devices emit in a single spatial and spectral mode, the L-I curve shows several discontinuities, with the distance between the discontinuities decreasing with increasing current. These discontinuities are associated with small wavelength jumps of 0.63 Å , which correspond to the value for the longitudinal mode spacing (see inset of Fig. 1).
Fig. 3 Normalized lateral far-field patterns at power levels ranging from 200 to 600 mW in steps of 100 mW.
In general, the mode closest to the DBR peak reflectivity has the lowest modal loss and hence becomes the lasing mode. As the injection current and therefore the temperature increases, the modes shift to longer wavelengths, but at a faster rate than the reflectivity spectrum. As the lasing mode moves away from the DBR peak reflectivity, there comes a point where the adjacent mode has a higher reflectivity. At this point the wavelength suddenly jumps to a new mode, resulting in a 0.63 Å decrease in the emission wavelength. Between these mode jumps, the wavelength can be tuned continuously. The inset of Fig. 1 shows the possible different stable modes , indicated as solid circles. For this specific example, the DBR has a cleaved end facet (semiconductor–air interface) located at half the grating period. Other DBR cleave locations will be treated below.
The optical spectrum for different power levels is shown in Fig. 2. The full-width at half maximum (FWHM) value of the oscillating mode is limited by the resolution of the optical spectrum analyser (0.2 Å ). The wavelength tuning rate with current is 0.016 Å/mA and with temperature is 0.7 Å/ºC measured between 100 and 500 mW. The laser emits in a single spatial and spectral mode with a sidemode suppression ratio exceeding 40 dB for power levels below 550 mW. For power levels above 550 mW the sidemode suppression ratio decreases to about 30 dB at 600 mW.
Fig 4. Calculated distribution of effective DBR reflectivity
Fig. 3 shows the normalised lateral far-field distribution at output powers of 200, 300, 400, 500 and 600 mW. The FWHM of the lateral far-field distribution is about 12º at 200 mW and, due mainly to thermal effects, increases with increasing power. The shape distortion, especially at higher output power is due to the insertion of an attenuator plate in front of the detector. The high power levels required the attenuation of the output beam to prevent detector saturation. The FWHM vertical far-field beam divergence of 28º (not shown) remains almost unchanged over the entire power range.
The residual reflection originating from the semiconductor to air interface at the DBR termination affects the overall reflectivity and depends on the exact location with respect to the grating period. To study this effect we calculated the reflectivity of 101 DBRs including end facet reflectors with varying distance between the facet and the DBR grating period. It is assumed that in practice the cleaving process is random, and produces a similar distribution. Without an end reflector, the fabricated DBR (power) reflectivity is 93.3%. The inclusion of an end reflector increases the mean value of the effective DBR reflectivity to 95%. Fig. 4 shows the effective reflectivity distribution of the 101 devices. It can be seen that about 45 devices have an effective DBR reflectivity between 96 and 97%.
Singlemode, single-frequency DBR lasers have been fabricated with power levels exceeding 700 mW. The emission spectrum of the DBR device tunes quasi-continuously with tuning coefficients of 0.016 Å/mA and 0.7 Å/ºC . The sidemode suppression ratio is greater than 30 dB over the entire range of operation. Theoretical investigation of the effect of random cleaving positions within the DBR showed that the mean value of the effective DBR reflectivity is increased by 1.7% compared with no end reflector.
The authors thank S. McWilliams, D. Phan, J. Threadgill, H. Huo and P. Le for processing the laser devices.
© The Institution of Engineering and Technology 2007 23 April 2007
Electronics Letters online no: 20071185
M. Achtenhagen and N.V. Amarasinghe (Photodigm Inc., 1155 E. Collins Blvd., 200, Richardson, TX 75075, USA)
G.A. Evans (Department of Electrical Engineering, Southern Methodist University, Dallas, TX 75275, USA)
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