Linglin Jiang, Martin Achtenhagen, Nuditha Vibhavie Amarasinghe, Preston Young,

and Gary Evans

Photodigm Inc., 1155 E. Collins Blvd., 200, Richardson, TX 75081, USA

Novel In-Plane Semiconductor Lasers VIII, edited by Alexey A. Belyanin, Peter M. Smowton, Proc. of SPIE Vol. 7320, 72310F copyright 2009 SPIE, doi: 10.1117/12.807872

 

Abstract

Spectral and output power data of distributed Bragg reflector lasers emitting in the technologically important wavelength range from 780 nm to 1083 nm are presented. These devices are fabricated in a single molecular beam epitaxy growth step, and the gratings are defined by holographic interferometry.  Spectral dependencies on the grating and gain section lengths are systematically investigated.  Experimental data for the side-mode suppression ratio, mode spacing, and thermal wavelength shift are given for devices emitting in the near infrared wavelength range between 780 nm and 1083 nm.

Keywords: Distributed Bragg reflector, laser diode, semiconductor laser.

 

1. Introduction

High-power distributed Bragg reflector (DBR) lasers are used in numerous applications requiring both single-frequency and wavelength tunability over a few tenths of a nanometer. Typical applications are Raman and absorption spectroscopy [1], narrow line pumping of rare earth-doped amplifiers [2], short pulse generation [3,4], and in numerous sum or difference frequency applications including mid-infrared and long wavelength (THz) generation as well as visible light generation by second harmonic conversion [5,6]. Table 1 shows typical applications for single-frequency lasers in this wavelength range.

Table. 1. Laser application

l(nm)	High-Power Single-Frequency DBR Laser Application
780	Atomic clocks, cold atom physics, quantum encryption, frequency standards, optical data storage
852	Atomic clocks, Cs+ absorption spectroscopy, test instruments
920	Frequency doubling
976	Frequency stabilized Er+/Yb+ pump sources; narrow band pumping of Yb+ fiber amplifiers
1064	Frequency doubling, display, imaging, YAG replacement, seeding of fiber amplifiers
1083	Hyper-sensitive He+ Magnetometers, Lung imaging

 

The ability to fabricate these devices in a single molecular beam epitaxy (MBE) growth step with gratings defined by holographic interferometry significantly reduces device complexity and processing time, thus minimizing manufacturing costs.  The DBR lasers currently produced by this method cover the wavelength range from 780 nm to 1083 nm, with output power from 40 mW to around 700 mW, and have outstanding single-mode, high-quality beam spectra. 

The spectral properties of DBR lasers are of major importance to the above-mentioned applications, and may vary significantly from one specific application to another.  Therefore, proper understanding of the interplay between the DBR and the gain sections is a critical requirement for the overall device design.

In this paper, we systematically investigate the physical properties of the grating and gain sections to study the impact on the emission spectra and device performance.  The detailed spectral characteristics of the 780 nm, 974 nm, 1064 nm and 1083 nm DBR lasers are not limited to these specific wavelengths and may serve as guidelines for other wavelengths. 

This paper is organized as follows:  In section 2, the basic epitaxial structures and manufacturing steps for the DBR laser diodes are presented.  Section 3 contains experimental and simulated spectral property data for DBR laser devices.  The mode spacing and wavelength shift dependence on device temperature and current, and the Side Mode Suppression Ratio (SMSR) dependence on temperature are investigated for 1083 nm, 974 nm and 780 nm DBR lasers.  In section 4, DBR laser performance data are given, including power, spectral, and lifetime data for the studied wavelength range.  The summary and acknowledgement for this work are presented in Section 5.

 

2. DBR Laser Structure and Fabrication

 

The DBR laser structures are processed from MBE grown AlGaAs (InGaAs, GaAs)/AlGaAs/GaAs epitaxial wafers. AlGaAs, GaAs, and InGaAs are chosen for the quantum well (QW) material, depending on the desired wavelength.  Graded Index Separate Confinement Heterostructure (GRINSCH) and cladding layers are composed of AlGaAs.  The exact Al concentration and layer thickness vary for the different wavelength structures to provide the appropriate QW barrier height required to minimize leakage current.  Prior to DBR laser fabrication, photoluminescence (PL) mapping is performed to determine the exact aluminum mole fraction and uniformity across each MBE-grown wafer.  The averaged values are then used to derive the exact ridge and grating etch depths required for each structure during fabrication.  The DBR laser fabrication steps are summarized below.

The DBR lasers are processed in two major steps similar to the method in [7].  In the first step, the gain section including the p-metal contact is processed, and in the second step, the grating section is defined.  The ridge waveguide of the gain section is formed by a wet etch followed by a self-aligned process for electrical isolation on either side of the ridge.  Following p-metal (Ti/Pt/Au) evaporation, a window in the Si₃N₄ covering the grating region is opened, followed by the deposition of a thin (~500 Å) layer of photoresist.  The gratings are formed by holographic lithography with the designed grating period, followed by dry etching to the designed grating depth.  Approximate grating periods for each laser wavelength are given in Table 2.  Second-order grating periods are used for 780 nm and 852 nm DBR lasers, while first-order gratings are used for longer wavelength lasers.

Figure 1. Schematic of typical DBR laser device with SEM photos of key features.  

	Table 2:  DBR grating period for each wavelength
	Bragg Wavelength (nm)	Grating Period (Ǻ)
		1st order	2nd order
	780	-	2350
	852	-	2600
	920	1370	-
	976	1460	-
	1064	1610	-
	1083	1640	-

After the gratings are defined, a 5 mm wide confinement ridge is formed and etched into the grating sections.  Following the confinement ridge, a final Si₃N₄ layer is deposited to serve as a protective film on the gratings.  Final processing steps include wafer thinning to ~100 mm and n-side metallization.  The wafer is then cleaved into bars, passivated, and optionally antireflection (AR) facet coated.  Bar level characterization includes light output and voltage (LIV), spectrum, and beam profile as a function of injection current.  Finally, after individual device cleaving and packaging, the devices are once again characterized. The spectral wavelength variation with respect to current and temperature is significantly reduced after device mounting.  Figure 1 shows key elements of the final device structure including the gain ridge, p-type metallization, and the DBR grating section. 

3. Spectral Properties of DBR Lasers

3.1 Wavelength Tuning

Typically, DBR laser diodes emit in a single spectral mode located at the wavelength closest to the DBR peak reflectivity, thus requiring the least amount of gain to reach threshold.  With increasing injection current, the lasing mode moves toward longer wavelengths.  As this dominant lasing mode shifts farther away from the DBR peak reflectivity, an adjacent cavity mode will eventually possess higher reflectivity, and the emission wavelength then jumps to the new mode at a shorter wavelength.  This behavior is illustrated in the curves of Fig. 2a for a 778 nm DBR laser.  This plot shows that for any fixed temperature, the wavelength for this specific laser slowly increases over an approximate 25 mA current range followed by a “mode-hop” back to a shorter wavelength.  The exact wavelength change upon mode-hop depends on the mode spacing, which itself depends on the effective cavity length and the group refractive index as discussed in a section 3.2. 

Figure 2 (a): Wavelength characteristics for a 778 nm DBR laser with increasing current at temperature steps from 15°C to 50°C; (b) Constant wavelength  contours for a 1083 nm DBR laser with increasing current from 10°C to 50°C.

 

Figure 2b shows contours of constant wavelength versus injection current and temperature for a 1083 nm DBR laser.  The two vertical curves overlaid in Fig. 2b show paths of continuous, mode-hop free wavelength tuning for constant laser output power.  These curves show that through proper current and temperature control, linear wavelength tuning without mode hops can be achieved over large current and temperature ranges.  This behavior is easily predictable, and accurate wavelength selection at predefined power levels and temperatures can be achieved by proper gain and grating section design.  The cavity modes shift within the DBR reflectivity curve according to changes in the gain section length and refractive index due to carrier concentration changes (via Kramer’s-Kronig relation) and/or by temperature changes.  For the 778 nm and 1083 nm DBR lasers shown in Fig. 2, the thermal tuning coefficients are 0.6Å/C° and 0.7Å/C° respectively, while the injection current tuning coefficients are 0.026 Å/mA and 0.017 Å/mA, respectively. The thermal and current tuning rates are similar for other wavelength DBR lasers after AR coating and packaging. 

One key feature of the DBR lasers presented in this work is that the current tuning rate is approximately half that of typical distributed feedback (DFB) lasers in this wavelength range [8].  The grating sections of these DBR lasers are passive and thus require no complex regrowth step as needed for DFB lasers. Furthermore, this design property significantly reduces current-induced heating effects in the grating section, thus minimizing the wavelength shift due to a specific current-induced heat load.  One additional feature of the passive DBR grating section is that propagation of current-induced defects in the laser epitaxial structure are minimized, thus allowing far greater wavelength stability and repeatability over operating lifetime when compared to DFB lasers.

The wavelength shifts and mode jump characteristics for the DBR lasers discussed in this section assume CW (DC) operating conditions with respect to temperature and current changes (carrier effects at or above threshold become fixed due to gain clamping.)  Under pulsed conditions, the temperature effects may be generally neglected for sufficiently short pulse lengths (typically less than 300 ns) 

 

3.2 Mode spacing

A DBR laser can be described as a conventional Fabry-Perot (FP) laser with a wavelength selective mirror (filter.)  The cavity mode frequency spacing in a FP laser is determined by the length of the cavity (c/2L), where the length is simply the effective optical length of the laser itself.  For a DBR laser, an additional length must be included to account for penetration into the DBR grating.  This additional length is commonly referred to as the effective DBR length and is typically defined as the position where the electrical field decays to its 1/e value.  This value can be estimated by the simple analytical expression [9]: 

                                                      (1)

where k is the coupling coefficient determined by the physical grating parameters such as grating depth, and L_DBR is the physical length of the DBR section.  For the lasers described in this work, this value of L_DBR is typically around 100 mm.  The mode spacing can then easily be defined as: 

                                           (2)

where l is the lasing wavelength, n_g,eff and  n_DBR,eff  are the effective group refractive index of the gain and DBR sections respectively, and L_gain  isthe physical length of gain section.  Assuming that n_g,effand n_DBR,effare equal, Eq. (2) can be written as:

                                                                           (3)

where n_g is the group index and L_tot is the total of the gain and effective DBR section lengths.  

 

Figure 3: Experimental data plotted as inverse mode spacing as a function of gain length for a 974 nm DBR laser.

 

A set of 974 nm DBR lasers were tested by progressively cleaving back the gain sections from an initial length of 1500 mm down to 500 mm.  The mode spacing was then measured and averaged over several mode jumps.  Figure 3 shows this data plotted as inverse mode spacing as a function of the gain section length.  This data is then used with Eq. 3 to derive a group refractive index of 4.587.  This group refractive index can then be used to calculate the mode spacing for arbitrary gain section lengths as part of final device design optimization.  The same experimental process can be applied for other wavelength DBR lasers to evaluate the group index as part of device analysis.

 

3.3   Side Mode Suppression Ratio

In general, the side mode suppression ratio (SMSR) of a DBR laser diode is determined primarily by the grating and cavity design, but also depends on spectral and spatial hole-burning effects. For simplicity, the secondary effects of spectral and spatial hole burning are neglected in the following discussion.  Also for the following analysis, the wavelength of the lasing mode is assumed to be aligned with the main lobe of peak DBR reflectivity.  The threshold gain difference DG between the lasing mode and the nearest side mode is [10]:                        

(4)

where R_DBR(l₀) and R_DBR(l₁)  are the intensity reflectivities of the DBR grating at the lasing wavelength (l₀) and the side mode wavelength (l₁), respectively.  With increasing gain length, the threshold gain difference decreases, which decreases the SMSR.  The DBR laser spectral dependency on gain length has been investigated for 974 nm DBR lasers with fixed grating lengths in [10].  Here we show the effects of DBR length on the SMSR for 1083 nm DBR devices with fixed gain lengths.  The DBR sections were systematically cleaved back from 800 mm to 200 mm lengths in 200 mm steps.  The SMSR was then measured at 50 mA, 100 mA and 200 mA current levels as shown in Fig. 4a.

Figure 4 (a): Measured SMSR data as a function of DBR length for injection current I = 50 mA, 100 mA, and 200 mA for a 1083 nm DBR laser;  (b):  Calculated curves for DG as a function of DBR length  for grating refractive index contrast values of Dn = 0.001, 0.0015, 0.002, and 0.0025.

 

Figure 4a shows that the SMSR increases nonlinearly with increasing current for constant DBR lengths, and is attributed to spectral and spatial hole burning effects to be discussed in a later work.  The calculated curves of Fig. 4b show DG as a function of DBR length for different values of Dn corresponding to the refractive index modulation within the DBR grating section.  The differences in Dn are attributable to the physical parameters of the DBR grating section including grating etch depth and duty cycle.  By comparing the shapes of the plots in Fig. 4, we find that the experimental SMSR data is best fit with a Dn range from 0.001 to 0.0015, (very close to the original targeted design value for the 1083nm devices in this work.)  It is generally observed that high quality single mode operation with SMSR > 40dB can be easily achieved by designing the laser for DG values greater than 0.5 cm^-1. 

                                                                                     

4. Measured DBR Laser Performance

4.1   Output power

High power single mode lasers are in great demand in numerous applications as outlined in Table 1.  We have fabricated DBR lasers to meet this need in several different wavelength and power ranges using the methods described in the previous sections.  Table 3 gives the typical slope efficiency and maximum power currently available for each DBR laser wavelength.

Table. 3. DBR laser performance.

l (nm)	Grating Order	Slope Efficiency (mW/mA)	Single-mode Maximum CW output Power  (mW)
780	2nd	0.8	110
852	2nd	0.85	150
920	1st	0.68	310
976	1st	0.72	425
1064	1st	0.72	550 (>700)
1083	1st	0.85	550

 

For both 1083 nm and 1064 nm DBR lasers, power outputs greater than 500 mW are routinely achievable while still maintaining SMSR over 30 dB after passivation and AR coating.  Output powers of more than 700 mW have been obtained from selected 1064 nm DBR laser devices.  The 852 nm and 780 nm DBR lasers have only been recently developed, and are still undergoing design and process optimization.  The targeted output powers for the 852 nm and 780 nm series of DBR laser diodes are 300 mW and 200 mW respectively.  The primary output power limitation for the single mode 780 nm DBR laser diodes has been due to the onset of catastrophic optical damage (COD).  Ongoing optimization tasks include refinement of the epitaxial design to reduce the optical power density at the laser output facets by increasing the mode field spot diameter, and the incorporation of non-absorbing mirrors (NAMs) at the output facets.

 

4.2   Emission spectrum

The DBR laser emission spectrum is determined primarily by temperature and injection current, and to a lesser degree, the final laser packaging.  It should be noted that under high power operation, large operating currents can contribute significantly to the overall device temperature, thus proper thermal management is required to maintain spectral stability.  Figure 5 shows the spectral output performance of 778 nm and 1065 nm DBR lasers.  The temperature tuning characteristic for a 778 nm device is shown in Fig. 5a. The output wavelength as a function of power (injection current) for a 1065 nm DBR laser maintained at a constant heatsink temperature of 25 ^oC is given in Fig 5b.  Figure 5 shows that the DBR laser diodes studied in this work exhibit high quality single mode operation (SMSR > 40dB) over a wide range of operating current and temperature.

Figure 5.  Normalized output spectra for (a) 778 nm DBR laser diode as a function of increasing temperature from 15 ^oC to 55^oC in 10^oC steps; and (b): 1065 nm DBR laser diode as a function of increasing output power from 200 mW to 500 mW in 100 mW steps at a constant 25 ^oC heatsink temperature

 

4.1   Reliability 

The reliability of semiconductor lasers depends on many factors.  These include the original epitaxial design, as well as strain and defects that occur during the epitaxial growth.  Additional strain and defects can be introduced during the laser processing steps, specifically during photolithographic, dielectric, and metal layer deposition.  Secondary failure mechanisms include stresses introduced in packaging, and lastly, the operating conditions and environment.

The degradation mechanisms that determine the lifetime of a semiconductor laser can be accelerated by operating the device at elevated temperatures. The time-to-failure (TTF) of a device at a nominal operating temperature (T₂) can be calculated from the TTF of a device under an elevated temperature (T₁) by using the Arrhenius equation [11]:

                                                         (5)

where k is Boltzman’s constant, and E_a is the activation energy, which is determined by the failure mechanism causing the degradation.  Generally the range of E_a is from 0.2 eV to 0.7 eV.

The TTF of a device is also dependent on the drive current at which the device is operated.  This relation is often described mathematically by [11]:

                                                                (6)

where I_i is the drive current corresponding to TTF_i, and n is the current degradation factor, typically ranging from 1.5 to 2.0.

The lifetime testing for the DBR lasers described in this work is performed under constant power conditions where the drive current is automatically adjusted to maintain a constant emitted optical power level over the lifetime of the device.  The operating current is recorded over several thousand hours, and the rate of change in the required current is used to project the lifetime of an individual device.  Once the time of failure for each device is estimated for a given set of operating conditions, the mean time-to-failure (MTTF) is calculated under the assumption that the failure times follow a statistically log-normal distribution.

Current lifetime data is available for the 780 nm, 920 nm, 1064 nm, and 1083 nm series of DBR laser diodes.  Figure 6 shows typical lifetime data for 780 nm and 1083 nm DBR laser diodes.  These curves indicate that the DBR lasers exhibit robust lifetime performance, with operating temperatures ranging from 25 ^oC to 85^oC under constant power mode test conditions.  Table 4 summarizes the estimated MTTF for DBR lasers of each wavelength for room temperature and constant power operation.  The MTTF at other operating temperatures and power levels can easily be derived from Eq. (5) and Eq. (6).  

 

Figure 6:   Sample lifetime data for (a) 780 nm DBR lasers operating with 60 mW constant output power at 25°, 45° and 65°C; (b) 1083 nm DBR lasers operating with 100 mW constant output power at 25°, 55° and 85°C having over 8500 hours of operation.  

 

Table. 4: DBR laser lifetime data.