DFB Laser Tunes Over Four WDM Channels
Etalon-stabilized diode laser module offers engineers 6 nm of tunability with better than 40 pm accuracy.
By: Kristin Lewotsky
A frequency-stabilized indium gallium arsenide phosphide (InGaAsP) diode laser module operating in continuous-wave (CW) mode offers 6 nm of tunability, providing coverage over seven channels on the International Telecommunications Union (ITU) grid for wavelength division multiplexing (WDM) (see Figure 1). The module incorporates a high-power distributed feedback (DFB) laser mounted on a double-stage thermo-electric cooler for frequency tuning. Michel Cyr and colleagues at Nortel Advanced Technology developed the device, which uses a previously-reported etalon-based wavelength-locking technology to provide wavelength stability over seven channels, with an accuracy of ±40 pm (±5 GHZ).
Figure 1: Tunable laser mounted on double-stage cooler incorporates ball and gradient-index (GRIN) lenses in the front optical train.
Although the experimental unit operates over the range from 192.5 THz (1557.363 nm) and 193.1 THz (1525.24 nm), similar modules can be designed to operate anywhere in the erbium-doped fiber amplifier pass band around 1550 nm. Eventually Nortel plans to commercialize the technology, offering network integrators a series of seven-channel modules that span the entire ITU grid.
Module design
The laser incorporates a high-power distributed-feedback indium gallium arsenide phosphide/indium phosphide (InGaAsP/InP) ridge waveguide structure grown by low-pressure metal-organic chemical vapor deposition (MOCVD). The strained-layer structure incorporates five quantum wells.
The front-facet optical train incorporates an aspheric lens, a semi-double isolator and a gradient-index lens to condition the output beam prior to launching it into the fiber. The use of polarization-maintaining fiber makes the 14-pin butterfly packaged unit usable with external modulators.
Tuning and wavelength stabilization
By varying the temperature range from -10 to +50°C, the laser can be tuned over the entire range. The double-stage cooler consists of a 29-couple top cooler mounted on a 69-couple bottom cooler. The cooler can maintain a temperature difference between the hot and cold plates of at least 80° C.
Although coarse tuning is accomplished by adjusting temperature, the design performs long-term frequency stabilization via a current-based feedback loop. At the back facet of the laser, a ball lens produces nearly collimated output that passes onto an etalon filter that acts as frequency discriminator (see Figure 2). To provide multi-frequency stabilization, the module incorporates a solid etalon made of a fused-silica spacer with 60%-reflective mirrors.
Figure 2: Etalon acts as multifrequency filter to pass a "comb" of frequencies spaced to the 100 GHz ITU grid.
A pair of closely-spaced photodetectors acting as apertures detect the slightly divergent beam of light transmitted through the solid etalon. In the case of spectrally stable output, both detectors intercept the same intensity of light. A spectral shift in laser output causes an intensity shift in the beam passed by the etalon, which unbalances the levels of light intercepted by the photodetectors. The resulting current variation can be used in a feedback loop to stabilize the transmitter wavelength.
Thermal compensation
The etalon filter is designed with a spacer element that allows it to pass a series of wavelengths. The refractive index of the material changes as a function of temperature, with a temperature coefficient of -1.25 GHz/°C (-0.01 nm/° C). The etalon is mounted on the same submount as the laser, which has an operating range of -10° C to 50° C. This thermal variation would introduce a 75-GHz (0.6-nm) uncertainty into the wavelength stability of the module.
To minimize thermally-induced inaccuracy, the designers incorporated a thermal compensation scheme that minimizes the etalon filter temperature coefficient effect. The filter is designed with a free spectral range of around 90 GHz (0.72 nm) instead of 100 GHz (0.8 nm). With a change in temperature, the set, or comb, of spectral peaks passed by the filter shifts to lock to the next 100 GHz grid (see Figure 3).
Figure 3: In the thermal compensation scheme, the comb shifts to intersect the previous or the next ITU-grid frequency. The comb of the two photodetectors currents over frequency is drawn at three different temperatures, corresponding to 100 GHz steps in laser frequency. With temperature change, the comb of etalon transmission peaks shifts toward higher or lower frequency depending on a decrease or increase of temperature.
For example, to decrease the laser frequency by 100 GHz (0.8nm) we have to increase the temperature by 8° C, which would shift the etalon filter transmission curve by 10 GHz (0.08 nm). So, the free spectral range of 90 GHz plus the thermal shift of the filter give you the next ITU channel.
Performance
The laser linewidth is less than 4 MHz and the absolute frequency accuracy is within ± 5 GHz (40 pm) over 7 channels on the 100 GHz grid (see Figure 4).
Figure 4: Plot of absolute accuracy as a function of frequency for five different modules shows good frequency stability over multiple channels.
By adjusting the photodetector gain, the designers expect to improve the frequency accuracy ±0.02 nm (2.5 GHz). The output power is +13 dBm (20 mW). The module fiber power varies by +1/-1.5 dB over channels (see Figure 5).
Figure 5: Plot of power variation as a function of frequency for five different modules demonstrates good power stability.