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DBR Lasers

This is a continuation from the previous tutorial - performance of DFB semiconductor lasers.


The DBR laser provides an alternative scheme in which the frequency dependence of the distributed-feedback mechanism is utilized to select a single longitudinal mode of an FP cavity.

In contrast to DFB lasers, the grating in a DBR laser is etched outside the active region [refer to Figure 7-1 in the DFB semiconductor lasers tutorial]. In effect, a DBR laser is an FP laser whose mirror reflectivity varies with wavelength; lasing occurs at the wavelength for which the reflectivity is maximum.

In this tutorial we briefly describe the operating principle and characteristics of DBR lasers.


1. Design Issues

Historically DBR lasers were developed in parallel with DFB lasers. A problem inherent in the DBR laser is that when the unpumped active material is used to etch the grating on both ends, optical losses inside the DBR region are high and the resulting DBR reflectivity is poor.

The problem of material loss can be overcome by using a material for the distributed Bragg reflector that is relatively transparent at the laser wavelength. For InGaAsP lasers the InP substrate or a quaternary cladding layer (with a band gap higher than that of the active layer) can be used for this purpose.

However, in this case the distributed Bragg reflector and the active region form two separate waveguides, and transfer of the optical mode between them leads invariably to coupling losses that reduce the effective reflectivity of the DBR.

The design of a DBR laser involves minimization of the coupling losses, and many coupling schemes have been used for this purpose.

If \(C_0\) is the power coupling efficiency between the DBR and the active waveguide, the effective amplitude-reflection coefficient of the DBR is given by


where \(r_\text{g}\) is the amplitude-reflection coefficient of the DBR at the junction of the two waveguides. The subscript \(\text{g}\) emphasizes the grating origin of the reflectivity.

The power coupling efficiency \(C_0\) appears in Equation (7-5-1) since coupling losses occur twice during each reflection.



2. Theory

An expression for \(r_\text{g}\) can be obtained using the analysis of the DFB Laser Theory section of the DFB semiconductor lasers tutorial.

Consider a distributed Bragg reflector of length \(\bar{L}\). A forward-propagating wave is incident at its surface where \(z=0\) and excites the counterpropagating waves \(A(z)\) and \(B(z)\) inside the reflector. The coupled-wave equations (7-3-15) and (7-3-16) [refer to the DFB semiconductor lasers tutorial] govern their propagation, and the general solution is given by Equations (7-3-23) and (7-3-24) [refer to the DFB semiconductor lasers tutorial].

The amplitude-reflection coefficient \(r_\text{g}\) is obtained from


where \(q\) and \(r(q)\) are given by Equations (7-3-21) and (7-3-22) [refer to the DFB semiconductor lasers tutorial].

If we use the boundary conditions \(B(\bar{L})=0\) at the other end of the reflector, we obtain from Equation (7-3-24) [refer to the DFB semiconductor lasers tutorial] the relation


Using Equation (7-5-3) in Equation (7-5-2), we obtain


This expression can be written in terms of \(\kappa\) and \(\Delta\beta\) using Equation (7-3-22) [refer to the DFB semiconductor lasers tutorial]; after some simplifications, it becomes


where, similar to Equation (7-3-33) [refer to the DFB semiconductor lasers tutorial],


The imaginary part of \(\Delta\beta\) is positive to account for the material losses inside the DBR medium, and \(\alpha_\text{g}\) is the corresponding power-absorption coefficient. The parameter \(\delta\) given by Equation (7-3-34) [refer to the DFB semiconductor lasers tutorial] accounts for the detuning of the laser wavelength from the Bragg wavelength.

Figure 7-18 shows the wavelength dependence of the power reflectivity \(|r_\text{g}|^2\) and the phase \(\phi\) of a reflector such that \(\alpha_\text{g}\bar{L}=0.1\).

The reflectivity is maximum at the Bragg wavelength (\(\delta=0\)), and a 50% reflectivity can be obtained even at the relatively small value for \(\kappa\bar{L}\) of \(1\).

Of course, the coupling loss \(C_0\) in Equation (7-5-1) would reduce the effective DBR reflectivity by \(C_0^2\).


Figure 7-18.  Power reflectivity and phase of a distributed Bragg reflector as a function of detuning from the Bragg wavelength for two values of \(\kappa\bar{L}\).


The threshold gain and the longitudinal modes of a DBR laser can be obtained in a manner similar to that discussed for FP lasers in the threshold condition and longitudinal modes of semiconductor lasers tutorial.

We assume for simplicity that the two DBRs are identical and can be described through the same effective amplitude-reflection coefficient \(r_\text{eff}\).

If we set the net change in the field amplitude after one round trip to unity, the threshold condition, similar to Equation (2-3-6) [refer to the threshold condition and longitudinal modes of semiconductor lasers tutorial], becomes




and is the mode-propagation constant.

Equating the modulus and the phase on the two sides of Equation (7-5-7), we obtain



where we have used Equation (7-5-1) and \(\phi\) is the phase of \(r_\text{g}\).

As in the case of an FP laser, it is convenient to define the DBR loss as


Equation (7-5-9) then simply becomes \(\bar{\alpha}=\alpha_\text{DBR}\). If we use Equation (7-3-6) [refer to the DFB semiconductor lasers tutorial] for \(\bar{\alpha}\), the material gain at threshold is given by


The phase equation (7-5-10) determines the longitudinal modes of a DBR laser. In contrast to FP lasers, however, the modes are not equally spaced since \(\phi\) depends on the detuning \(\delta\) of the mode wavelength from the Bragg wavelength.

Similarly, the DBR loss \(\alpha_\text{DBR}\) is different for different longitudinal modes and the lowest-threshold mode occurs at the wavelength for which \(\alpha_\text{DBR}\) is the smallest.

Figure 7-19 shows the longitudinal modes and their respective threshold gains for a DBR laser.

Equation (7-5-5) was used to obtain \(|r_\text{g}|\) and \(\phi\) as a function of the detuning \(\delta\). The normalized DBR loss \(\alpha_\text{DBR}L\) was then obtained using Equation (7-5-11).

In obtaining Figure 7-19, we choose \(C_0\) equal to \(1\) after assuming no coupling losses.

The lowest-threshold mode occurs close to the Bragg wavelength since the DBR reflectivity peaks at that wavelength.

In contrast to DFB lasers, DBR lasers do not exhibit a stop band in their longitudinal-mode spectra. This is because no distributed feedback occurs inside the DBR laser cavity. Conceptually, a DBR laser is just an FP laser whose end mirrors exhibit frequency-dependent reflectivity.


Figure 7-19.  Longitudinal modes and their respective threshold gains for a DBR laser. Variation of the amplitude-reflection coefficient of the distribute Bragg reflector with the detuning \(\delta\bar{L}\) is used to obtain the phase and the DBR loss. The bottom part shows graphical solutions (points of intersection) of the phase condition; various diagonal lines correspond to different integer values of \(m\) in Equation (7-5-10). Solid dots in the top part show the DBR loss, or equivalently the threshold gain, for various DBR modes.



3. Emission Characteristics

DBR semiconductor lasers operating at 1.55 μm were developed during the 1980s in parallel with DFB lasers. A buried-heterostructure design is used in order to reduce the threshold current and to maintain single-mode operation in the lateral and transverse directions. The first-order gratings are generally formed in the DBR regions with a coupling coefficient \(\kappa\) of \(\sim100\text{ cm}^{-1}\).

The emission characteristics of DBR laser are similar to those of DFB lasers. Relative to DFB lasers, the threshold current of a DBR laser is generally high because of coupling losses, which increase \(\alpha_\text{DBR}\) given by Equation (7-5-11). For the same reason, the differential quantum efficiency is also relative low. Several different designs have been used to reduced the coupling losses in DBR lasers, and threshold currents below 100 mA are readily achieved.

The performance of a DBR laser is comparable to that of a DFB semiconductor laser as far as the spectral and dynamic properties are concerned. The longitudinal mode closest to the Bragg wavelength has the lowest threshold gain (see Figure 7-18) and becomes the dominant mode.

Because of the significant gain margin (\(\sim8\text{ cm}^{-1}\)), the other longitudinal modes are suppressed by about 30 dB relative to the main mode. These side modes remain suppressed under high-speed modulation even though the mode suppression ratio degrades somewhat as the modulation depth increases.

In a structure, referred to as a bundle-integrated-guide DBR laser, side modes remained suppressed by more than 30 dB for modulation frequencies as high as 2 GHz while, at the same time, the frequency chirp was below 0.2 nm.

The spectral line width under CW operation is also quite small for DBR lasers. In one device, a line width of 3.2 MHz was measured at a relatively low output power of 1.5 mW. The line width of a DBR laser depends on the design details such as whether one or two Bragg reflectors are used and whether the Bragg reflectors are adjacent to the active region or separated by a passive region. A line width of 560 kHz was realized in one DBR laser whose structure was properly optimized.

A new type of DBR laser has been developed to improve the performance of DBR lasers. This structure, referred to as the distributed reflector (DR) laser, is a combination of DFB and DBR structures since two separate gratings of different pitches are fabricated in the active and passive sections of such a device.

The main advantages of this structure are (i) almost 100% coupling efficiency between the active and passive sections, (ii) a high differential quantum efficiency because of a low internal loss, and (iii) a low chirp under direct modulation because of a reduction in the effective line-width enhancement factor.

Threshold currents in the range 30-35 mA and differential quantum efficiencies in the range 15-20% have been realized in such lasers. The effective line-width enhancement factor is found to be smaller by about a factor of 2, resulting in low chirp under direct modulation and a low linewidth under CW operation.

However, it is difficult to obtain line widths below 1 MHz. The use of an MQW active region has resulted in a line width of 743 kHz in one device that also exhibited an MSR in excess of 40 dB.

DBR lasers with an MSR as high as 58 dB have been fabricated by using the technique of chemical beam epitaxy. In another record performance, the use of a strained-layer quantum-well InGaAs active layer has resulted in 0.98-μm DBR lasers capable of operating at power levels up to 42 mW with a differential quantum efficiency as high as 47%.

DBR lasers operating in the visible region near 0.6 μm have also been fabricated by using a strained InGaP quantum well as the active layer. In a new class of DBR lasers the light is emitted in a direction perpendicular to the wafer surface. Such DBR lasers are known as surface-emitting lasers.

In a different approach, hybrid-type DBR lasers have been developed. These lasers are coupled-cavity type in the sense that an external Bragg reflector is coupled to a conventional multimode semiconductor laser.

In one scheme, a silicon-chip Bragg reflector is used to provide the distributed feedback. Line widths below 0.5 MHz have been obtained by such a hybrid approach, and even smaller line widths are possible. Such DBR lasers are also capable of producing picosecond optical pulses through mode locking.

In another approach the grating is etched directly onto a fiber that is coupled to the multimode laser. Such a fiber-DBR laser has been used to produce 18.5-ps pulses at a repetition rate of 2.37 GHz. The main disadvantage of such hybrid DBR lasers is related to their not having a monolithic design.

It is possible to design a monolithic extended-cavity DBR laser by etching the grating directly onto a passive waveguide formed on the same substrate used for fabrication of the laser.

In one 1.55-μm device, the 0.6-mm long Bragg mirror was separated from the 0.7-mm active region by a 4.2-mm passive waveguide, resulting in a 5.5-mm-long DBR laser. All three components were formed on the same InP substrate to preserve the monolithic nature of the device. This laser produced 20-ps optical pulses at an 8.1-GHz repetition rate through active mode locking.

In another implementation, a 1.2-mm-long MQW DBR laser provided a 200-kHz line width at 17-mW operating power. The laser has a low threshold current (8 mA) and could be operated up to 28 mW.



Coherent and multichannel communication systems often require semiconductor lasers which not only operate in a single longitudinal mode with a narrow line width but whose wavelength can also be tuned over as wide a range as possible.

The next tutorial will discuss multisection DFB and DBR lasers, which have been developed to meet these requirements. 



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