Symmetrical linear external-cavity lasers

When angled-stiipe semiconductor amplifiers are used as gain media in external-cavity laser configurations the amplifier must be coupled to two external cavity sections to allow laser oscillation. AR coated diodes require only one such section as do chips produced with only one angled facet^^. The diode amplifier is highly absorbing to its own radiation when unpumped and a requirement for modelocking with this cavity type is that the diode is placed centrally between the two external-cavity mirrors. That is, the cavity should be symmetrical or balanced (see figure 3.8). The tuning element allows the laser output to be tuned over the large gain

bandwidtli of the amplifier, and also acts to restrict the oscillation bandwidth which is desirable for good modelocked performance. Possible tuning elements can be étalons, prisms and diffraction gratings. For modelocking the total cavity length is normally in the range of a few centimetres to -Im . For experimental configurations, long cavity lengths are normally chosen to aid flexibility and harmonic modelocking is utilised to keep the repetition rate within acceptable limits.

Total cavity length, L

Tuning element

^ Output ►

R -100^ Amplifier Coupling

optics Figure 3.8 Symmetrical extemal-cavity semiconductor laser.

Output coupler

The standard modelocking technique for low frequency systems is to apply a

subthreshold DC bias and superimpose onto this an RF signal of frequency equal to the external cavity mode spacing (or some multiple thereof). For a Im long laser the cavity mode spacing ( ^ ) , where L is the optical path length) is 150MHz. However, in the case of a balanced cavity, modulation at 150MHz would mean that the intracavity pulse would experience gain only on alternate passes through the diode amplifier and hence the output pulses would be seriously

degraded. For this reason the modelocking frequency of the balanced cavity is chosen using

the relation,

f _nL

resonance criterion as for modelocked ring lasers (L in this case defines the optical path length of the perimeter) where the pulses also collide in the gain medium.

3.5.1 Lensed fibre cavities

The laser configurations studied here utilised microlensed fibre external-cavitiesl^'^ instead of the more conventional microscope objective/bulk mirror (or diffraction grating) free-

space external cavities. The reason for this is twofold; firstly the small dimensions of the fibre microlenses can allow access to the output from angled-ridge amplifiers without resorting to complicated heat-sink headers, and secondly, the fibre medium is more compact than free-space cavity sections because it can be wound into a reasonably tight coil (diameter >50mm) without affecting its transmission. Minors can be directly buttedü^ onto the end of a cleaved fibre without the introduction of significant loss or alternatively some type of integrated fibre minor/filter may be used. An integrated fibre-grating reflector was used in part of this work (see section 3,8) but in general the cavities incorporated a bulk-optics cavity section to facilitate tunability and more importantly bandwidth control (see figure 3,9). Prism and étalon tuned configurations will feature in this chapter along with the fibre grating cavity. The more attractive

bulk diffraction-grating loaded external-cavity laser was also extensively used and will be described in chapter 4.

(a) (b) (c) '' I ... MO Output MG! _{Drawn tapered} microlensed fibre M y . ... 0 (d) ^specular , Y reflection

Figure 3.9 Experimental realisations of balanced external-cavity lasers incorporating (a) étalons, (b) prism, (c) diffraction grating, and (d) an integrated fibre grating reflector.

3.5.2 CW operating characteristics of symmetrical external-cavity lasers incorporating angled-ridge semiconductor amplifiers.

The laser cavity as illustrated in figure 3.9(a) was configured using an angled-ridge (10°) waveguide InGaAsP amplifier. The light/current char acteristics measured for the laser is shown

in figure 3.10, where curve (a) was recorded with the external cavity blocked and curve (b) obtained for the unblocked cavity. The solitary angled-ridge amplifier could not sustain laser oscillation for current levels in excess of 6-7 times the threshold cunent of an identical Fabry- Perot device (~40mA). The fluorescence intensity as a function of injection current was typical of an ASE laser, but single-sided feedback was observed to affect the features of the fluorescence spectrum. Figure 3.11 shows such a fluorescence spectrum which is characterised by a large period spectral modulation which only appears when significant optical feedback is present as was described in section 2.5. This modulation influenced the ftee-running oscillation spectrum of the laser (see figure 3.12(a)) where oscillation on several peaks of the modulated gain spectrum were observed with finer detail being evident within each cluster.

Ph

0 _{80, I(mA)}

Figure 3.10 Light/current characteristics of the symmetrical fibre extemal-cavity laser for (a) the free-space section blocked, and (b) unblocked.

I=80mA

1418 1628 X (nm)

Figure 3,11 Fluorescence specti'um of angled-ridge amplifier with single-sided external-cavity feedback

1564 1524

1509 ► ₁₅₅₆

X (nm)

Figui-e 3.12 (a) Free-running spectrum of symmetrical external cavity laser, and (b) after insertion of two étalons (0.5 and 0.2mm)

The total emission spectrum can be viewed as a superposition of a series of spectral modulations on the essentially bell-shaped material gain spectrum of the semiconductor amplifier.

These are (i) the ~1 Inm period feedback induced modulation, (ii) the Fabry-Perot modulation due i

to eitiier the semiconductor amplifier and/or reflections from the fibre microlenses [The measured | period was 0.66nm and the expected periodicity for the amplifier cavity and the cavity formed between the two fibre lenses was 0.67 and 0.65nm respectively therefore the measurement resolution was insufficient to provide an unambiguous source for this modulation], and (iii) the modulation due to the external cavity itself which would be '-150MHz and hence was

unresolvable in these measurements.

By inserting two étalons into the free-space cavity section a single Fabry-Perot mode defined by the amplifier/fibre lens subcavity could be isolated (see figure 3.12(b)). The most

suitable étalons were found to have thicknesses of 0.5 and 0.2mm, and by rotating the étalons some degree of wavelength tunability was provided.

3.6 Modelocked symmetrical external cavity semiconductor lasers