WO2001073905A1

WO2001073905A1 - Wavelength tuning in external cavity lasers

Info

Publication number: WO2001073905A1
Application number: PCT/GB2001/001299
Authority: WO
Inventors: Tingdi Liao; Raymond John Chaney
Original assignee: Renishaw Plc
Priority date: 2000-03-25
Filing date: 2001-03-23
Publication date: 2001-10-04
Also published as: GB0007160D0

Abstract

An external cavity laser system includes a laser diode (1) which emits an outgoing beam of light (2) into the cavity, the cavity contains a collimating lens (3) which receives and collimates the beam (2), a rotatable prism (6) through which the collimated beam passes, and a diffraction grating (4) onto which the beam is directed after passing through the prism. The diffraction grating in a first embodiment is designed to generate a minus one order beam which is reflected back to the laser diode and a zero order beam which forms the output (5) from the cavity. Rotation of the prism enables the angle of incidence of the outgoing beam on the diffraction grating to be varied to tune the wavelength of the output beam.

Description

WAVELENGTH TUNING IN EXTERNAL CAVITY LASERS

The present invention relates to external cavity lasers .

For the purposes of this specification a laser should be taken to mean any device which produces a coherent beam of light .

Conventional external cavity lasers are generally made in two configurations.

In a first configuration known as the Littrow configuration, the system consists of a laser, for example a diode laser, with an anti-reflection coating on its front face, a collimating lens and a rotatable diffraction grating. The zero order beam from the diffraction grating forms the output of the external cavity laser.

Wavelength selection, line narrowing, and frequency stabilisation are achieved in this configuration by rotation of the diffraction grating. The minus first order diffraction beam is used as the optical feedback beam by properly setting the angular orientation of the diffraction grating. One disadvantage of this configuration is that, in frequency tunable systems, the orientation of the output beam is dependent on the lasing frequency.

In the second configuration known as the Littman-Metcalf configuration, the system may also consist of a diode laser with an anti-reflection coating on its front face, a collimating lens, and a fixed diffraction grating. However, in addition, a rotatable mirror is positioned opposite the diffraction grating to receive the diffracted beam and reflect it back to the diffraction grating. Also in this configuration the zero order beam is the output and the minus first order beam is used as a feedback beam in order to provide for wavelength and frequency tuning.

In both of these configurations the high sensitivity of the wavelength and frequency of the device to the angular position of the grating makes it very difficult to achieve fine tuning of the lasing wavelength, and requires that the diffraction grating should be mounted in a very stable configuration once it has been properly oriented.

An object of the present invention is to overcome this difficulty of wavelength tuning in external cavity lasers .

According to the present invention an external cavity laser system having an output beam of tunable wavelength comprises a laser which emits an outgoing beam of light and an external cavity connected to the laser into which the outgoing beam from the laser passes, said cavity containing optical elements positioned in the path of the outgoing beam for producing from the outgoing beam both an output beam from the cavity and a feedback beam which is directed back towards the laser, wherein the optical elements include a wavelength tuning device, the wavelength tuning device comprises a dispersive element and an angularly adjustable deflecting element which produces a change in the angle at which at least one of the outgoing and feedback beams strikes the dispersive element .

In one embodiment of the invention the laser is a laser diode the beam from which is collimated by a collimating lens and the angularly adjustable deflecting element is positioned in the path of the outgoing beam between the collimating lens and the dispersive element and varies the angle at which the outgoing beam strikes the dispersive element.

In an alternative embodiment the dispersive element produces a deflected beam which is partially reflected from a semi-transparent mirror to form the feedback beam, and the angularly adjustable deflecting element is disposed between the dispersive element and the mirror.

The angularly adjustable deflecting element is preferably a prism which is rotatable about an axis normal to the plane containing the prism angle. With this arrangement the change in the angle of deflection is less than the angle of rotation of the prism.

The dispersive element is preferably a diffraction grating.

Embodiments of the invention will now be described with reference to the accompanying drawings in which: Fig 1 shows a diagrammatic layout in plan view of the optical components used;

Fig 2 shows the optical components of Fig 1 designed into a practical system;

Fig 3 is an exploded view of the components of Fig 2 ; and

Fig 4 is a diagrammatic representation of the components of an alternative embodiment of the invention.

Referring now to the drawings, in Fig 1 the optical components of an external cavity diode laser system in the Littrow configuration are shown. The system comprises a diode laser 1 orientated so that the long axis (-100 microns) of the output aperture is vertical, and the short axis (-2 microns) is horizontal.

In this configuration the laser emits an outgoing beam 2 which is vertically polarised.

Owing to diffraction, the beam divergence is large in the horizontal plane, and small in the vertical plane.

A lens 3 is placed in front of the diode to collimate the light in the horizontal plane. This is relatively easy to achieve because the light source appears as a "point" source in the horizontal plane. In the vertical plane the light source is extended and is difficult to collimate .

A prism 6 is mounted between the lens 3 and a diffraction grating 4 with its prism angle a in the horizontal plane. The prism is rotatable about a vertical axis so that the light emerging from the prism may be angularly deviated in the horizontal plane.

In this embodiment the properties and configuration of the diffraction grating 4 are so chosen such that it can only produce two output beams from a single incident beam i.e. the zero order diffracted beam (or reflected beam) 5, and the minus one order diffracted beam. No other diffraction beams exist in this configuration.

The groove direction of the diffraction grating is in the vertical plane, and the plane of incidence is in the horizontal plane. In this example, as shown in Fig 2, the zero order diffracted beam 5 is reflected such that its direction of propagation is approximately perpendicular to the direction of propagation of the output beam of the diode laser, and forms the output beam of the external cavity laser system.

The minus one order diffracted beam is reflected back towards the diode laser (Littrow configuration) .

The angle of the back reflected light, in the horizontal plane, is dependent upon its wavelength. In the horizontal plane the light is well collimated and hence can be focused by the lens. Only light in a narrow range of wavelengths will be focused by the lens into the output aperture of the diode laser. Consequently, only these wavelengths will be amplified by the diode laser.

Owing to the polarisation of the light incident on the grating, only about 20% of the incident light is reflected back towards to the diode laser; the rest is diffracted and forms the output beam.

The change in the beam deviation angle produced by rotation of the prism is a weak function of the angle of rotation σ of the prism. The change in the angle of the beam emerging from the prism, caused by rotation of the prism, produces a corresponding change in the incident angle θ of the beam on the diffraction grating, thus allowing fine tuning of the wavelength of the reflected beam returning to the diode laser.

Referring now to Figs 2 and 3 the physical layout of the optical components in the external cavity 10 is shown. The diode laser 1 is mounted to the front face of the cavity, and the beam 2, after collimation by lens 3, passes through the rotatable prism 6 to impinge on the diffraction grating 4. The diffraction grating is mounted in the cavity at an angle to the axis of the beam 2, so that the minus one order diffracted beam is retro-reflected along the axis to the diode laser. The output beam 5 passes out of the casing through an outlet aperture 12 in a side face of the cavity. A further aperture 14 is provided in the side face of the cavity for an electric cable 16 to provide power and control for the diode laser.

The rotatable prism 6 is mounted on a stand 20 (Fig 3) and is rotatable about the vertical axis A by a PZT drive mechanism 22 controlled by a controller 24.

The angle of the prism is determined according to the required wavelength tuning sensitivity and tuning range. Preferably the prism is set at an offset angle to the beam axis so that rotating the prism allows one to tune the wavelength in either direction (λ+Δλ) .

The diffraction grating is orientated at an angle determined by the equation: 2Λsin(θ)=λ for the Littrow configuration, where Λ is the groove spacing of the grating.

In this system the grating is set at the nominally correct angle and remains stationary and the prism rotates during wavelength tuning. The incident angle change (Δθ) is produced by the beam deviation when rotating the prism, yielding the wavelength tuning range (Δλ) .

Rotating the prism while keeping the grating stationary is equivalent to rotating the grating to tune the wavelength in the conventional Littrow devices. However, using the prism for wavelength tuning has significant differences from rotating the grating. These novel characteristics are described below in more detail .

As shown in Fig 1, the inclusion of the prism in the external cavity diode laser can dramatically alter the wavelength tuning sensitivity. This can be seen from theoretical modelling of the system. Analysis of the beam deflecting characteristics of a prism shows that the beam deviation angle, χ, produced by the prism depends strongly on the angle, , but only weakly depends on the beam incident angle, σ, with respect to the normal of the first surface of the prism.

A large angle variation of σ produces only a small angle variation of χ. Therefore the beam incident angle (θ) on the diffraction grating varies with χ, which is weakly dependent on σ. The typical frequency tuning sensitivity S=Δλ/Δσ is around 0.066 nm per milli-radian for a prism angle of =6° . A factor of between 20 and 100 reduction in sensitivity of wavelength tuning can be achieved without much difficulty by using this new method.

The prism can also be used to fine-tune the emission frequency by effecting small variations in the cavity optical length. This is because a rotation of the prism will change the geometry of the cavity, both in the glass and in the air, and thus change the optical path length of the cavity.

In the conventional ittrow configuration, the transverse displacement of the collimating lens due to mounting error is rather critical to the frequency stability of the device. Thus, a collimating lens of high image quality together with a proper mounting of the lens with balanced thermal stability is required. Another advantage of the invention is that this critical mounting tolerance can be greatly relaxed on set up when a prism is used in the extended cavity.

In a conventional external cavity diode laser, when the collimating lens has a transverse displacement (Δs) along the direction perpendicular to the grating line during set up due to mounting tolerances, an angular deviation of the collimating beam will result. This angular deviation will correspondingly result in a frequency or wavelength shift.

However, in the proposed external cavity diode laser with a prism, the prism can be used to correct the beam angular deviation resulting from the transverse misalignment or displacement of the collimating lens thus relaxing the mounting tolerances of the laser diode/lens combination. The prism angle and material of the prism can be varied to optimise the wavelength tuning characteristics (tuning sensitivity and tuning range) and frequency stabilisation.

Similar considerations apply to the mounting of the diffraction grating.

In the conventional Littrow configuration, the grating is required to be mounted with high angular accuracy, since the emission frequency is highly sensitive to grating angle.

The inclusion of a prism in the laser cavity means that the diffraction grating does not need to be mounted to high precision at the outset. This is because the prism permits very precise wavelength tuning after installation of the diffraction grating. Any slight initial error in the grating angle (e.g. ±10 arc minutes) can easily be corrected by coarse angular tuning of the prism. However, once the angles have been established it is still important that the mounting of the grating is stable.

This invention has been described in the Littrow configuration. It is also applicable to the Littman- Metcalf configuration and other external cavity diode lasers incorporating diffraction gratings.

The rotatable prism may also be used to control the lasing wavelength of the external cavity laser system, for example, by using a portion of the beam to obtain feedback from an air refractometer . Referring now to Fig 4 an alternative embodiment is shown. Components of this embodiment which are the same as (or similar to) those of the previous embodiment are given the same reference numerals. In this embodiment the diode laser 1 produces an outgoing beam 2 which is directed into the external cavity where it passes through a collimating lens 3 before striking the diffraction grating 4.

In this embodiment however, the diffraction grating is a blazed grating so that only one diffracted beam 5 is produced. The beam 5 is directed through an angularly adjustable prism 30 which is rotatable about an axis A to a semi-transparent mirror 32 which allows a proportion of the light beam to pass through and to exit the cavity as the output beam 3 . The remainder of the light is reflected back towards the diffraction grating. The beam 2 produced by the diode laser 1 includes a range of wavelengths, and because the angle of the diffracted beam 5 is a function of the wavelength of the light, the diffracted beam spreads out as indicated by the dotted lines.

Only that part 5a of the beam 5 having a wavelength such that it strikes the mirror normally will be reflected back to the diffraction grating from which it is in turn reflected back to the laser diode where it is amplified. Thus that part 5a of the beam resonates within the cavity and concentrates the energy of the beam into a narrow band of wavelengths at the output of the cavity.

Rotation of the prism about the axis A alters the angle of the diffracted beam 5 as it passes through the prism and enables light of the desired wavelength to be directed from the diffraction grating normally onto the mirror. Thus although in the above-described embodiment the diffraction grating is fixed nominally at the correct angle, the rotation of the prism allows fine tuning of the wavelength of the output beam. Once the optimum angle of the prism has been found for the fixed position of the diffraction grating the prism may also be fixed. However, both the prism and the diffraction grating may be adjustably mounted.

The invention has been described with reference to a prism forming the tuning element. However, other equivalent elements may be used to achieve the same effect, for example, a diffractive optical element, or a transmission grating.

Further the dispersive element described is a diffraction grating but it is to be understood that other equivalent dispersive elements may be used in some embodiments, for example a prism, or a prism and mirror combination where a reflective dispersive element is required.

Claims

1. An external cavity laser system having an output beam of tunable wavelength comprises a laser which emits an outgoing beam of light and an external cavity connected to the laser into which the outgoing beam from the laser passes, said cavity containing optical elements positioned in the path of the outgoing beam for producing from the outgoing beam both an output beam from the cavity and a feedback beam which is directed back towards the laser, wherein the optical elements include a wavelength tuning device comprising a dispersive element and an angularly adjustable deflecting element which produces a change in the angle at which at least one of the outgoing and feedback beams strikes the dispersive element.

2. An external cavity laser system according to claim

1 wherein the laser is a laser diode and the optical elements in the cavity include a collimating lens for collimating the outgoing beam.

3. An external cavity laser system according to claim

2 wherein the angularly adjustable deflecting element is positioned in the path of the outgoing beam between the collimating lens and the dispersive element and varies the angle at which the outgoing beam strikes the dispersive element.

4. An external cavity laser system according to claim 1 wherein the dispersive element produces a deflected beam which is partially reflected from a semi- transparent mirror to form the feedback beam, and the angularly adjustable deflecting element is disposed between the dispersive element and the mirror.

5. An external cavity laser system according to claim 1 wherein the adjustable deflecting element is a prism which is rotatable about an axis normal to the plane containing the prism angle.

6. An external cavity laser system according to claim 1 wherein the dispersive element is a diffraction grating.