Tunable lasers handbook phần 9 pptx

384 Paul Zorabedian

7.1.1.5 Camera Lenses

There are at least three published reports on the use of camera lenses as col￾limators in ECLs. Heckscher and Rossi [57] reported the use of a TV camera

lens for intracavity collimation of a Littrow grating cavity, but gave no indication

of the feedback strength obtained. Sommers [58] evaluated several camera

lenses from f10.99 (25-mm focal length) to f12.0 (50-mm focal length). The

lenses gave only about 1% feedback when used with a grating, and it was con￾cluded that spherical aberration was responsible for the poor performance since

the lenses were not used in their intended geometry. Fleming and Mooradian

successfully employed camera lenses in an ECL [38]. They used 50-mm focal

length,fll.4 seven-element lenses. All air-glass surfaces were AR coated.

7.1.1.6 Ball Lenses

Glass spheres can be used to couple the gain medium to waveguide or fiber￾pigtailed external filters. However, the spherical aberrations are too great to be

useful for collimation in bulk optic cavities.

7.1.1.7 Lensed Fiber

Lensed optical fiber [59] can be used to couple the gain medium to fiber￾pigtailed external cavities. However. this method requires the fiber to be in very

close proximity to the facet, which gives rise to the danger of facet damage.

There is also a very high sensitivity of the coupling loss to lateral misalignment.

7.7.2 Optics for Beam Expansion and Shaping

7.1.2.1 Cylindrical Lenses

A cylindrical lens can be used in an ECL [60] to form a line illumination on

a diffraction grating. This implements a degenerate resonator in one dimension

and provides a high degree of angular misalignment tolerance while maintaining

high spectral selectivity. Critical to the success of this technique is the fact that

the cylinder axis can be inclined with respect to the optical axis at a large angle

to match the grating angle of incidence without introducing a large amount of

spherical aberration. This is because the cylinder lens has no power in this plane

and appears to be a tilted plate.

7.1.2.2 Prisms

The use of prism beam expanders allows the use of a compact, high-resolution

grating-tuned extended-cavity laser [61]. A particularly useful geometry is when the

apex angle 8, is cut so that

8, = 90" - tan-' (11) . (44)

where ri is the index of refraction of the prism material. For this choice of apex

angle, the output beam is normal to the exit face of the prism (which is the

8 Tunable External-Cavity Semiconductor Lmers 385

condition of maximum expansion) when the angle of incidence equals the Brew￾ster angle. The magnification of each prism is then equal to the index of refrac￾tion of the prism material, that is, M = 17.

7.2 Tunable Filters

The ideal filter for an ECL has a bandwidth that is less than the axial mode

spacing of the cavity and has 0-dB insertion loss at its peak. No real filter is

ideal, but a number of different types of wavelength-selective elements have

been used to tune external cavity lasers. The filters are grouped according to

whether they are actuated by mechanical means (e.g., have moving parts) or

electronically (no moving parts).

7.2. 7 Mechanically Tuned Filters

7.2.1.1 Diffraction Gratings

7.2.1.1.1 Types of Gratings

Diffraction gratings are the most common type of filter used in ECLs and

have arguably the best optical performance. A diffraction grating consists of a

large number of regularly spaced grooves on a substrate. The distance between

adjacent grooves is called the pitch. If the underlying substrate is reflective. then

we have a I;?jection gl-atiizg [Fig. 18(a)]. If the substrate is transmissive, then the

device is said to be a tl-ansmissiorz gmtiizg [Fig. 18(b)].

Diffraction gratings are also classified by the way in which they are manu￾factured. When the grooves are created by scribing with a ruling engine, the

device produced is a ruled mastel- grating. Relatively few masters are produced,

and these are rarely sold. The groove pattern of the master can be faithfully

Transferred by a contact process to a number of replica gratings, which are then

made available commercially (e.g.. by Milton Roy).

Diffraction grating groove patterns are also generated by exposing photo￾resist with the fringe pattern created bj two interfering beams of laser light,

Such gratings are called holographic and are also sold commercially (e.g., by

American Holographic).

7.2.1.1.2 Principle of Operation

When a beam of light is incident on a grating, each groove generates a dif￾fracted wavelet. For each wavelength component in the incident beam, the con￾structive interference of the diffracted components from each groove occurs at a

unique set of discrete directions called the diffraction oi-del-s of the grating.

7.2.1.1.3 The Grating Equation

grating equation:

The geometry of the diffraction pattern from a grating is governed by the

386 Paul Zorabedian

a [ sin oi + sin cp,) = n7~ , (45)

where a is the groove spacing (pitch). is the incident angle, 'p, is the diffracted

angle of the m'th order, and n7 is the order of diffraction. The diffracted light is

dispersed according to its spectral content. with different wavelengths appearing

at different angles. Differentiating the grating equation gives the angular disper￾sion D, which describes how much the diffraction angle changes as the wave￾length varies:

Diffraction gratings are usually used in first order in ECLs, that is. with ni = 1. The

zeroth-order (specular reflection) beam is sometimes used for output coupling.

The wavelength resolution of a grating-tuned external cavity is determined

by the angular dispersion multiplied by the acceptance angle for coupling back

into the gain medium active region. The angular dispersion can therefore be used

FIGURE 1 8

(reproduced with permission from Palmer [62]).

Types of plane diffraction gratings. (a) Reflection grating. (b) Transmission grating

8 Tunable External-Cavity Semiconductor lasers 387

as a figure of merit, but it must be remembered that the parameter of ultimate

importance is the grating resolution divided by the axial mode spacing of the

external cavity. (For a detailed description of multiple-prism grating dispersion.

see Chapier 2.)

7.2.1.1.4 Common Mountings

Diffraction gratings in external cavity lasers combine the functions of the fil￾ter and external mirror. In extended cavities, the light from the grating must be

retroreflected back into the gain medium. Two common retroreflecting mounting

geometries for diffraction gratings in extended-cavity lasers are the autocollima￾tion (Littrow) configuration and the grazing-incidence (GI) configuration.

7.2.1.1.4.1 Littrow Moztnting In the Littrow configuration [Fig. 19(a)], [he

angles of incidence and diffraction are equal: Oj = 'pl. The grating equation becomes

In this case the angular dispersion of the retroreflected beam is identical to that

of the diffracted beam and is given by

A typical angle of incidence for the Littrow configuration is Oi - 50".

7.2.1.1.4.2 Grazing-Zncidence Mounting In the grazing-incidence con￾figuration (Fig. 19b). the intracavity beam makes two passes at the grating. The

diffracted light from the second pass is a retroflection of the incident light from

FIGURE 1 c' Diffraction grating mountings. (a) Littrou. (b) Grazing incidence.

388 Paul Zorabedian

the first pass. Therefore, the angular dispersion of the retroreflected light is twice

that of the light diffracted on one pass:

The dispersion of the grazing-incidence configuration is therefore twice that of the

Littrow configuration for the same angle of incidence. In addition, the grazing￾incidence configuration is typically used with a much higher angle of incidence,

for example, 8, - 85".

7.2.1.1.5 Grating Efficieizcy

7.2.1.1.5.1 Blazed Gratings Blazing refers to an enhancement in effi￾ciency that is obtained at a particular wavelength when the grooves on the grat￾ing surface have a triangular shape. A simple explanation for this effect is that

when the specular reflection from the top surface of each groove coincides with

the direction of diffraction, the reflections reinforce the diffraction effect and the

efficiency is maximized. The wavelength h, at which this reinforcement occurs

is called the "blaze wavelength." The angle 8, of the top surface of the groove

with respect to the macroscopic surface of the grating is called the "blaze angle."

The terminology derives from the observation that a grating will light up or

"blaze" when viewed at the correct angle.

The blaze angle of ruled gratings is defined during the process of ruling the

master grating and is transferred to the replica. The simplest type of holographic

grating has a sinusoidal shape. However, after interferometric recording, the

grooves of holographic gratings can be shaped to approximate blazing by an ion￾beam milling process.

In a Littrow mounting the blaze condition is satisfied when the tops of the

grooves are perpendicular to the incident beam. The diffraction efficiency rises

as the angle of incidence is increased up to -8, and falls thereafter. This simple

description is only valid for low blaze angles (up to -10'). Working near 1, for

small blaze angles implies a small diffraction angle as well, so that k<a. This is

the regime of validity for scalar diffraction theory, in which the diffraction effi￾ciency is nearly independent of polarization.

7.2.1.1.5.2 Polarization Effects To obtain greater angular dispersion it is

necessary to use larger blaze and diffraction angles, which implies IL - a. This is

the regime of vector diffraction theory in which polarization effects become sig￾nificant. For blaze angles above -lo", the diffraction efficiency strongly depends

on the orientation of optical polarization with respect to the direction of the

grooves. A particularly useful regime for tuning ECLs is the range of blaze

angles from about 22" to 38". For this regime, there is a broad plateau of high

efficiency for €Il > 8, when the incident polarization is perpendicular to the