Tunable Semiconductor Lasers
This is a continuation from the previous tutorial - DBR semiconductor lasers.
Tunable semiconductor lasers are useful for many applications. Examples of their applications in fiber-optic communication systems are (i) wavelength-division multiplexed lightwave systems where optical signal at many distinct wavelengths are simultaneously modulated and transmitted through a fiber and (ii) coherent communication systems which require wavelength matching between the local oscillator and the transmitter laser.
The simplest kind of tunable semiconductor lasers make use of an external cavity formed by placing a grating at some distance from a multimode laser. Such external-cavity semiconductor lasers can be tuned over a wide range (up to 80 nm) by simply rotating the grating.
However, their use in practical lightwave systems is limited because of their non-monolithic design. For this reason considerable effort was spent during the 1980s to develop tunable multisection DFB and DBR lasers.
The basic idea behind the tunability of DFB and DBR lasers is simple. The operating wavelength of such lasers is determined by the etched grating through the Bragg wavelength given by \(\lambda_0=2\bar{\mu}\Lambda/m\), where \(\bar{\mu}\) is the mode index, \(\Lambda\) is the grating period, and \(m\) is the diffraction order of the grating [refer to Equation (7-2-1) in the DFB semiconductor lasers tutorial].
Even though the grating period \(\Lambda\) is fixed during grating fabrication, the laser wavelength can be changed by changing the mode index \(\bar{\mu}\). A simple way to change the mode index is to change the material index by injecting current into the grating region, since the refractive index depends on the injected carrier density. In general, the refractive index decreases with an increase in the carrier density, resulting in a shift of the Bragg wavelength (and hence the laser wavelength) toward shorter wavelengths with an increase in the injected current.
Several schemes can be used to design tunable DFB and DBR devices. For example, a DBR laser with one Bragg reflector, designed such that different currents can be injected into the active and Bragg sections allows wavelength tuning by varying the current in the passive Bragg section.
However, such two-section lasers allow only discrete tuning. Continuous tuning is achieved by using a multisection DBR laser shown schematically in Figure 7-20. It consists of three sections, referred to as the active section, the phase-control section, and the Bragg section.
Each section can be biased independently by injecting different amounts of current. The current injected into the Bragg section is used to change the Bragg wavelength through carrier-induced changes in the refractive index. The current injected into the phase-control section is used to change the phase of the feedback from the DBR through carrier-induced index changes in that section.
The laser wavelength can be tuned continuously over a range 5-10 nm by controlling the currents injected in the three sections. The performance of a three-section DBR laser with 80-mm-long phase-control section is shown Figure 7-21 where the measured device wavelength is plotted as a function of the phase-section current.
The current in the Bragg section is also adjusted by a small amount to maximize the sweep range. This laser is tunable continuously over more than 6 nm by this technique.
It is possible to extend the tuning range by reverse biasing the Bragg section. A tuning range of 22 nm was obtained in a three-section DBR laser by changing the current from -120 to 120 mA, where the negative and positive currents correspond to reverse and forward biasing of the Bragg section.
The physical mechanism behind the reverse-biased tuning is local heating of the Bragg section. Since the refractive index increases with an increase in the temperature, the laser wavelength shifts toward the red side, in contrast with the forward biasing that results in a blue shift because of the carrier-induced reduction in the refractive index.
A disadvantage of the thermal-tuning mechanism is that it is relatively slow, i.e., wavelength changes occur at a time scale of ~ 100 μs compared with the case of forward biasing where carrier-induced tuning occurs at a time scale of ~ 1 ns. It is nonetheless fast enough for some applications.
For applications in coherent communication systems the laser should exhibit a narrow line width. Considerable theoretical work has been done to understand the performance of multisection DBR lasers.
A formalism based on a Green's-function approach is generally used to calculate the line width since it allows inclusion of the effects of spatial inhomogeneities and spatial-hole burning.
The results show that the effective line-width enhancement factor of a tunable DBR laser depends on many device parameters and can change with the currents applied to the phase-control and Bragg section. As a result, the line width is expected to change considerably with wavelength tuning.
Figure 7-22 shows the line width as a function of wavelength for the same laser whose tuning curves are shown in Figure 7-21.
Although the line width is quite small (in the range 5-10 MHz), it becomes quite large just before the laser jumps to a neighboring mode. The reason behind this change in line width is related to a reduction in the Q factor of the cavity.
It should be noted that the current in the active region was kept constant in obtaining the data shown in Figure 7-22. A part of the increase in the line width is due to a decrease in the output power with tuning. Line-width variations are somewhat smaller if the tunable DBR laser is operated at a constant output power.
Modulation characteristics of multisection DFB and DBR lasers have also been studied. A useful property is their frequency modulation (FM) response. This can be quite uniform over a wide frequency range.
Because of their attractive characteristics such semiconductor lasers have been used in many transmission experiments.
The next tutorial introduces coupled-cavity semiconductor lasers.