FABRY-PEROT LASERS

The Fabry-Perot laser is conceptually just an LED with a pair of end mirrors. The mirrors are needed to create the right conditions for lasing to occur. In practice of course it is somewhat more complex than this - but not a lot. The Fabry-Perot laser gets its name (and its operational principle) from the fact that its cavity acts as a Fabry-Perot resonator

Fabry-Perot Filter. Light enters the cavity through a partially silvered mirror on the left and leaves it through a partially silvered mirror on the right. Only wavelengths that resonate within the cavity are able to pass through. Other wavelengths are strongly attenuated.

To understand the operation of the Fabry-Perot laser it is first necessary to understand the Fabry-Perot filter. The principle of the Fabry-Perot filter is illustrated in Figure 66. When you put two mirrors opposite one another they form a resonant cavity. Light will bounce between the two mirrors. When the distance between the mirrors is an integral multiple of half wavelengths, the light will reinforce itself. Wavelengths that are not resonant undergo destructive interference with themselves and are reflected away.

This principle also applies in the FP laser although the light is emitted within the cavity itself rather than arriving from outside.

In some sense every laser cavity is a Fabry-Perot cavity. But when the cavity is very long compared to the wavelength involved we get a very large number of resonant wavelengths all of which are very close together. So the important filtering characteristics of the Fabry-Perot cavity are lost.

We consider a laser to be Fabry-Perot when it has a relatively short cavity (in relation to the wavelength of the light produced). Wavelengths produced are related to the distance between the mirrors by the following formula:

Where: = Wavelength Cl = Length of the cavity x = An arbitrary integer - 1, 2, 3, 4... n = Refractive index of active medium

This is an extremely simple relationship. Notice here that the only other variable in the equation is the refractive index of the gain medium (dielectric) in the cavity. This is because we always quote the

wavelength as what it would be if the wave was travelling in a vacuum.51 Since the speed of propagation in the cavity is a lot lower than c (the speed of light) the wavelength is a lot shorter than it would be in free space. The adjustment factor is the refractive index.

51 A wavelength of 1500 nm in free space becomes a real physical distance of 1500/3.45 nm in InP which

equals 434.78 nm.

In practice, we cant make the laser so short that we restrict it to only one wavelength. We need some space for stimulated emission to amplify the signal and we are limited by the density of the power we can deliver to a small area. Typically the cavity length is between 100 and 200 microns (of the order of 400 wavelengths or so) although devices with cavities as short as 30 microns have been made. 1.490 1.494 1.497 1.5 1.503 1.507 1.510 Wavelength (nm)1.490 1.491 1.494 1.495 1.497 1.498 1.5 1.502 1.503 1.505 1.507 1.508 1.510

Resonance Examples

The Figures above shows two examples of typical resonances. On the left we have solved the equation above for a cavity 100 microns long, a wavelength of 1500 nm and a refractive index of 3.45 (InP). We can see that there are 7 wavelengths within 10 nm of 1500 nm where resonance may occur. On the right of the figure we can see the same solution but for a cavity 200 microns long. Here there are 13 possible resonant wavelengths. The longer the cavity (and the shorter the wavelength) the more resonant wavelengths we can find within the vicinity of our centre wavelength.

Resonance Modes in the Cavity of a Fabry-Perot Laser

Figure above (on the left) illustrates the principle of multiple resonant longitudinal modes in the FP cavity. We can get a number of resonant wavelengths provided the cavity length is an integer multiple of the particular wavelength.

On the right of the figure we see another problem. What if the sides of the cavity reflect light. What you get here are lateral modes forming which are also resonant and which can also lase! There are various ways of minimising or eliminating these lateral modes and this is discussed later. Transverse modes (vertical paths) cannot exist because the device is too thin in the vertical direction for multiple modes to exist. You could get a lateral mode that was completely side-to-side at right angles to the long axis of the device. You

could also get a vertical one of the same kind. However, lateral modes are suppressed as discussed later and there is not enough gain in the vertical direction for lasing to be sustainable.

Modes Produced in a Typical Fabry-Perot Laser

DISTRIBUTED FEEDBACK (DFB) LASERS

DFB Laser Schematic

When we want to use lasers for long distance communication we find that standard FP lasers have significant problems:

1. As seen above FP lasers produce many wavelengths over a spectral width of between 5 and 8 nm. Even if we are using the 1310 zero dispersion band or dispersion shifted fibre in the 1550 nm band there will still be some chromatic dispersion of the signal caused by dispersion being slightly different at the different wavelengths.

2. The mode hopping behavior of FP lasers gives rise to Mode Partition Noise as described in 2.4.3, Mode Partition Noise on page 67.

3. In Wavelength Division Multiplexed (WDM) systems we want to carry many multiplexed optical signals on the same fibre. To do this it is important for each signal to have as narrow a spectral width as possible and to be as stable as possible. Regular FP lasers have too great a spectral width for use in this application.

Distributed FeedBack (DFB) lasers are one answer to this problem. The idea is that you put a Bragg grating into the laser cavity of an index-guided FP laser. This is just a periodic variation in the RI of the gain region along its length.52 The presence of the grating causes small reflections to occur at each RI change (corrugation). When the period of the corrugations is a multiple of the

wavelength of the incident light, constructive interference between reflections occurs and a proportion of the light is reflected. Other wavelengths destructively interfere and therefore cannot be reflected. The effect is strongest when the period of the Bragg grating is equal to the wavelength of light used (first order grating). However, the device will work when the grating period is any (small) integer multiple of the wavelength. Thus only one mode (the one that conforms to the wavelength of the grating) can lase.

Early devices using this principle had the grating within the active region and were found to have too much attenuation. As a result the grating was moved to a waveguide layer immediately adjacent to (below) the cavity. The evernescent field accompanying the light wave in the cavity extends into the adjacent layer and interacts with the grating to produce the desired effect.

In principle a DFB laser doesnt need end mirrors. The grating can be made strong enough to produce sufficient feedback (reflection) for lasing to take place. However, in a perfect DFB laser there are actually two lines produced (one at each side of the Bragg wavelength). We only want one line. A way of achieving this and improving the efficiency of the device is to place a high reflectance end mirror at one end of the cavity and either an AR coating or just a cleaved facet at the output end. In this case the grating doesnt need to be very strong - just sufficient to ensure that a single mode dominates. The added reflections (from the end mirrors) act to make the device asymmetric and suppress one of the two spectral lines. Unfortunately they also act to increase the linewidth.

A schematic view of a DFB laser is shown in Figure 77 on page 113. DFB lasers are very effective and widely used but they have a problem with chirp. There are two main sources of chirp:

1. When the current is switched on the charge carrier (electron and hole) flux in the cavity changes very rapidly. This causes a change in the refractive index. A change in refractive index (of course) changes the resonant wavelength of the grating and the wavelength of the laser output changes (typically the wavelength gets longer) in well less than a single bit time.

2. During lasing the cavity heats up. This also happens very quickly (in a lot less than a bit time). This heating has two principal effects:

a. It causes the RI of the cavity to change. b. It changes the electron energy gap in the material.

In an FP laser (as distinct from a DBR or DFB laser) this change in the energy gap dominates other effects and is the predominant cause of chirp. In the DFB laser the energy gap change is irrelevant. This is because the energy gap covers a range of energies and the DFB resonant wavelength is determined by the grating spacing and the cavity RI. So long as the range of energies in the gap extends to cover the resonant wavelength then the device will lase.

This means that a DFB laser will chirp far less than an FP laser. This is because chirp in DFB lasers is caused by the effect of the change in RI. This effect is much smaller than the effect caused by the change in the energy gap (which dominates in FP lasers but doesnt affect DFBs).

Sometimes DFB lasers are constructed with a quarter-wave phase shift in the middle section of the grating as shown in Figure 78. This phase shift introduces a sharp transmission fringe into the grating reflection band. The fringe acts to narrow the linewidth of the laser significantly.

Phase Shifted Grating - Reflection Spectrum

Figure 79 shows the reflective characteristics of an unshifted and a shifted Bragg grating structure. Ascending values on the y-axis represent increasing percentage of reflection. The x-axis represents wavelength. The axes have not been scaled because the numerical values depend on the period and strength of the grating itself. The phase shifted case (on the right of the figure) shows that a narrow passband exists in the middle of the reflection band. This is caused by the quarter-wave phase shift. What happens is that the reflected waves from each end of the grating will be out of phase with each other and hence will destructively interfere.

DFB lasers have a number of significant advantages over FP types:

1. They can exhibit very narrow linewidths (of the order of 50 kHz). 2. They have quite low chirp as discussed above. 3. They typically have a very low Relative Intensity Noise (RIN).

Nothing however is completely without problems:

1. DBR lasers are extremely sensitive to reflections. Any reflection entering the cavity will disturb the lasers stable resonance. This causes a widening of the linewidth. To the extent that reflections returning from the outside vary (see 2.4.4, Reflections and Return Loss Variation on page 67) this can also be a significant source of noise. To minimise the effects of this problem DFB lasers are often packaged with an isolator integrated within the assembly. However, these dont always suppress all reflections and additional steps must be taken in system design to minimise the problem.

2. They are sensitive to temperature variations in two ways: a. The stable (average) temperature of the device has a very strong

influence on wavelength. Wavelength variation on a scale of many

seconds or longer doesnt have much detrimental effect on a single channel long distance communication system but it is a critical issue in WDM systems. The device requires temperature control for stable operation. This is usually provided by including a Peltier Effect cooler in the laser package.

b. During transmission (in even one bit time) the cavity heats up. If a long series of 1 bits are transmitted this can cause a significant wavelength shift on a time scale too short to be compensated by the Peltier cooler. This introduces a requirement that higher layer link protocols be balanced and spend (on average) as much time in the 0 state as in the 1 state.

3. Varying conditions produce significant fluctuations in laser output power. This is undesirable for many reasons. To counter this a PIN diode is often included in the laser package near the back facet. This diode picks up a small proportion of generated light from the transmittance of the back facet and provides input to a feedback loop for control of laser drive current.

4. They have a relatively high cost. As seen above, to get stable operation you almost always need temperature control, power control and optical isolation. All this adds to the cost.

VERTICAL CAVITY SURFACE EMITTING LASERS (VCSELS)

VCSELs55(also called microlasers) have been around in various forms since the late 1970s. However in 1991 there was a major development in construction techniques reported and in 1996 the first commercial devices became available.

It seems almost too obvious but when you build a laser you cant just arbitrarily decide on its structure. You are severely limited by material characteristics and available manufacturing technology. In previous sections we have discussed edge emitting lasers where you start with a flat substrate and use the techniques of chip manufacture to build a very thin, flat device that nevertheless has a relatively large area. VCSELs are different. Instead of emitting from the edge they emit from the surface. They are constructed by laying down a very large number (perhaps 500) of relatively thin layers of semiconductor material. The device emits light vertically through the stack of material layers. This is shown in Figure 92. As in any laser the overall structure is one of two end mirrors on each side of an active region which produces the light. The key points are as follows:

The mirrors are made of alternating layers of material of different refractive indices with carefully controlled thickness. The stack forms a Bragg grating (see 5.7, Diffraction Gratings on page 206) which is a wavelength-selective mirror.

The sides of the laser are formed by cutting the material out.

The laser dimensions can be such that no lateral modes are possible. In fact the laser is so confined that it forms a quantum well in which light behaves as individual photons rather than as waves or rays. Typical dimensions are about 12 microns in diameter (for single moded operation) and 20 microns (for multimoded operation). Experimental devices have been made and shown to operate with a diameter of only 3 microns.

The active region is very short compared to other types of semiconductor laser. This means that the mirrors have to have a relatively high reflectivity. (You need many passes through the amplifying medium to get enough amplification to sustain lasing.)

One of the big challenges is supplying power to the active region because it must pass through many mirror layers (junctions) first. This problem however has been solved.

VCSEL Structure

The operational characteristics of VCSELs combine many of the desirable properties of lasers and LEDs.

Typical coupled output power at present is somewhere around a milliwatt. This is very good for LAN type communications as we only want to transmit up to a maximum power of the (class 1) eye safety limit (-4 dBm). Typical LEDs are a lot lower in power and require the use of expensive InGaAs pin diode receivers. VCSELs in the LAN environment can work well with simple, low cost Si diode receivers.

Current VCSELs on the market offer one of two possible wavelengths: 980 nm or 850 nm. Again this is in the high attenuation window for glass fibre but quite adequate for distances of around 500 metres or less.

VCSELs with a large diameter (20 microns or so) have multiple transverse modes. This makes the device very suitable for use with multimode fibre. The big problem for lasers with multimode fibre is modal noise as discussed in 2.3.6, Modal Noise on page 51. The low coherence of output light produced by a multi-transverse mode VCSEL leads to insensitivity to mode selective loss and minimises the problem of modal noise.

A low divergence circular light beam is produced which allows for easy and efficient coupling to a fibre.

Typical VCSELs have very low threshold currents (less than 5 mA). Very low power dissipation and low modulation current requirements mean that special driver circuitry is not required.

VCSELs have very high modulation bandwidths (2.4 GHz has been demonstrated). This is well in excess of what can be achieved with much more expensive LEDs.

Devices are very stable and generally do not need a monitor photodiode or feedback power control as is conventionally needed for most communications lasers and high-end LEDs.

In 1998 four manufacturers have low priced VCSELs on the market. In the near future we could see them replacing LEDs completely for the LAN communications environment.