Tapered amplifier

A tapered amplifier (TA) is a semiconductor optical amplifier whose gain stripe flares from a single-mode ridge, typically 3–5 µm wide, into a trapezoidal section whose output aperture is a few hundred micrometres. A seed beam injected into the narrow end spreads by diffraction as it propagates, and the taper widens with it, so the intensity at the output facet stays below the damage limit while the beam keeps a single lateral mode. Seeded with 10–50 mW from an external-cavity laser or DFB diode, a TA chip typically delivers 1–3 W at wavelengths from the red to beyond 1 µm, with the seed's linewidth and tuning preserved. The combination is a master-oscillator power-amplifier (MOPA), used heavily in atomic physics: 780 nm Rb and 767 nm K laser cooling, and as a pre-amplifier ahead of frequency doubling.

Geometry

The ridge section, roughly 0.5–1 mm long, acts as a mode filter that rejects light not coupled into the fundamental lateral mode. The tapered section, typically 2–3 mm long, has a full taper angle of about 4–6°. That angle is chosen to match the free diffraction of the ridge mode inside the semiconductor. For a mode of waist radius $w_0$ = 1.5 µm at $\lambda$ = 780 nm in material of index $n \approx 3.4$, the diffraction half-angle is

$$\theta = \frac{\lambda}{\pi n w_0} = 0.049\ \text{rad} = 2.8^\circ,$$

a full angle of 5.6°. A 3 µm ridge feeding a 2.75 mm taper at a 6° full angle opens to

$$3\ \mu\text{m} + 2\,(2750\ \mu\text{m})\tan 3^\circ \approx 290\ \mu\text{m}$$

at the output facet. A taper narrower than the diffracting beam clips it; a wider one leaves unsaturated gain at the edges, which amplifies spontaneous emission and degrades beam quality. The output facet is anti-reflection coated, and the input facet is also coated, since the device must not lase on its own.

Gain and saturation

Small-signal gain of a TA chip can exceed 30 dB, but in normal use it runs deep in gain saturation: 20 mW in and 2 W out is 20 dB. In saturation the output depends weakly on seed power, so a factor-of-two drop in seed power typically costs much less than a factor of two in output, which makes the MOPA tolerant of seed fluctuations. Without a seed, the chip emits broadband amplified spontaneous emission of tens to hundreds of milliwatts. Part of the ASE co-propagates with the amplified signal as a pedestal a few nanometres wide, which is usually a small fraction of the power in saturated operation but can matter for spectroscopy near resonances.

Beam quality and output optics

The output is near-diffraction-limited in the slow axis, often with $M^2$ between about 1.1 and 1.5 and 70–85% of the power in the central lobe, and single-mode in the fast axis. The beam is strongly astigmatic: the fast axis diverges from the output facet, while the slow axis appears to diverge from a virtual source inside the chip, displaced from the facet by roughly the taper length divided by the refractive index (for a 2.75 mm taper and $n$ = 3.4, about 0.8 mm). Collimation therefore needs an aspheric lens followed by a cylindrical lens, and good beam quality after the optics depends on correcting that astigmatism. Coupling the output into single-mode fiber typically achieves 50–70% efficiency because of the side lobes and residual non-Gaussian structure, which is why mode matching receives the most alignment effort.

Pitfalls

A TA must be seeded before current is applied and the current removed before the seed is blocked. With no seed, the full gain acts on ASE and on any reflection returning to the chip, and the resulting intensity at the narrow input facet can destroy it. For the same reason an optical isolator, usually 30–60 dB, sits after the output and often a second one protects the seed laser from counter-propagating ASE. Input coupling must match the ridge mode; misalignment by a micrometre or two reduces output sharply and raises the ASE fraction. The chip dissipates several watts in a millimetre-scale footprint and needs a stable, well-sunk mount with temperature control.

Common questions

How does a tapered amplifier differ from a broad-area laser?

Both have wide output apertures, but a broad-area laser diode oscillates on many lateral modes in its own cavity, while a TA amplifies one injected mode whose lateral profile is set by the ridge filter. The TA's output inherits the seed's linewidth and tuning, while its lateral beam quality is set by the ridge filter and taper.

Can a tapered chip be used as a laser?

Yes. With a reflecting rear facet the same geometry forms a tapered laser, which emits watts with good beam quality, and with a grating in the ridge section it becomes a single-frequency tapered DBR laser. The MOPA arrangement is chosen when the seed's tunability and narrow linewidth are needed.

References: J. N. Walpole, "Semiconductor amplifiers and lasers with tapered gain regions," Opt. Quantum Electron. 28, 623 (1996); L. A. Coldren, S. W. Corzine, M. L. Mašanović, Diode Lasers and Photonic Integrated Circuits 2nd ed. 2012; Siegman, Lasers 1986.