Distributed Feedback (DFB) lasers are commonly used as tunable lasers over a range of a few nanometers. This is generally achieved by adjusting the temperature of the chip, thanks to a driver that can also monitor the current flowing through the DFB. This is a straightforward way of obtaining a tunable laser for a narrow range, typically one or two nanometers. However, temperature tuning can lead to unexpected behaviors which can be critical in many applications.
A distributed feedback (DFB) laser is a type of laser diode where the active region of the device is periodically structured as a grating. The structure builds a one-dimensional interference grating (Bragg scattering) providing optical feedback for the laser at a specific wavelength corresponding to the period of the grating. The device has multiple axial resonator modes, but there is typically one mode which is favored in terms of losses. Side modes could remain typically below -30/40 dB.
Figure 1: DFB laser. A one-dimensional interference grating is structured inside the active region.
Current DFB can operate in the range of 0.6 μm to 3 μm with typical output powers around some tens of milliwatts. The linewidth is typically from one to a few hundred MHz.
DFB lasers are commonly used as tunable lasers over a range of a few nanometers. This is generally achieved by adjusting the temperature of the chip or by monitoring the injected current. This is a useful way of obtaining a tunable laser for a narrow range and, due to the large free spectral range, without mode hops typically over one or two nanometers.
However, temperature tuning can lead to unexpected behaviors which can be critical in many applications. The limit of the mode-hop free has to be well determined, anticipating possible performance decrease over time, especially when the DFB is used for continuous back and forward tuning.
How does the tuning by temperature work?
The change in lasing wavelength by controlling the chip temperature or injected current both mainly result in a temperature change in the active layer of the lasers.
The lasing mode is shortened (resp. lengthened) with a decrease (resp. increase) in effective grating pitch introduced by the decrease (resp. increase) in the optical path (physical length x refractive index) due to the temperature decrease (resp. increase).
There is a quasi-linear relationship between temperature and center wavelength. The wavelength coefficients are typically between 0.1 nm/K and 0.03 nm/k depending on the wavelength and technology, i.e. few GHz/k.
Setup for temperature tuning
In order to tune the wavelength of a DFB laser, a setup can be used to maintain a stable or defined temperature using a thermo-electric temperature controller. Most laser diode applications use Thermoelectric (TE) coolers based on the Peltier Effect to maintain a steady temperature. TE modules are semiconductor “heat pumps” that move heat from one side of the device to the other. Depending on the direction of the current through the TE cooler, you can either heat up or cool down a laser diode. 15-40°C are typical recommended value for the DFB operation, which can be easily achieved by a commercial Peltier system.
Precautions to be observed regarding wavelength tuning with temperature
Unfortunately, with many of the low-end DFB lasers available on the market, mode-hopping or competing modes can occur over the specified tuning range. With such low quality DFB, you can often find a combination of temperature and injected current which provides your expected wavelength in single-mode regime, but it may be difficult to perform wide tuning without facing modes hopping. Hopefully, excellent quality DFB lasers can also be found but are much expensive.
To determine or verify the exact operating range and the spectral quality of your DFB you need to use an instrument which gives you the possibility to:
- Measure the true spectrum and not only the average wavelength, as it is the case with a Wavelength meter
- Discriminate two narrow laser modes at few tens of pm from each other
- Have a high-rate (or at least a representative rate of your application) measurement so as to see if the two modes co-exist.
For example, we checked an 852 nm DFB laser which could be considered of very high quality, and used it over a range slightly beyond its specified operating conditions. The scan was performed between 851.200 nm and 853.180 nm by tuning the Tset (kΩ) between 25.5kΩ and 5.4kΩ, corresponding to chip temperatures of 5°C and 40°C when the ambient temperature is 24°C. The recommended range of operating temperatures is 15°C–40°C. The 5°C–15°C tuning range was therefore out of specification.
Figure 2: (a) Measurement with ZOOM Spectra of an 852 nm DFB laser. Forward current 140 mA – Chip temperature 20°C: within the recommended temperature limits.
(b) Low-rate measurement (100Hz) with ZOOM Spectra of an 852 nm DFB laser. Forward current 140 mA – Chip temperature 7.9°C: outside recommended conditions. Two modes are measured 0.094 nm apart.
The spectrum in Figure 2 is the result of a low-rate (100Hz) measurement and one could deduce that there were two competing modes. However, when the measurement was performed at a relatively high rate (> 1,000 Hz), one can observe that these were hopping modes that never co-existed!
Another case where the DFB is not single-mode is when side modes are over the dynamic you need for your application.
Moreover, the PI curve may exhibit a non-linear behavior when increasing temperature. Indeed, lasing occurs at a wavelength for which the gain has a peak and the cavity a resonance. The gain peak may move at 0.4 mm/°C whereas the mode resonance peak shifts by 0.03 nm/°C to 0.1 nm/°C. Then if the gain and the mode resonance peaks move with temperature at different rates, at one point the lasing may be degraded and the power low. Usually, the increase in temperature causes a decrease in the optical power of the laser output.
Another important warning concerns the lifetime. Thermal stress can induce mechanical stress due to the different thermal expansion coefficients of the components. This can cause the growth of crystal defects or put a strain on the connections inside the laser device. Both effects can shorten the life time of the laser device.
This tunability of DFB lasers is useful for spectroscopy applications, laser diode pumping of solid state lasers and erbium-doped fiber amplifiers for instance. The quality of tunabilty may be very good with wide real mode-hope free range. However, it is always recommended to be critical about the quality of your DFB and to perform tests with a laser spectrum analyzer.