WO2005008852A2

WO2005008852A2 - An extended cavity diode laser

Info

Publication number: WO2005008852A2
Application number: PCT/GB2004/003017
Authority: WO
Inventors: Clemens Friedrich Kaminski; Iain Stewart Burns; Johan Frederik Hult
Original assignee: Cambridge University Technical Services Limited
Priority date: 2003-07-14
Filing date: 2004-07-13
Publication date: 2005-01-27
Also published as: WO2005008852A3; GB0316448D0

Abstract

A diode laser has a diffraction grating acting as output coupler and a semiconductor laser diode structure with an emission facet arranged to form an extended cavity with the diffraction grating. Mounting means are arranged to permit rotational movement of the diffraction grating with respect to the emission facet for wavelength selection. The mounting means is arranged to permit bulk translation of the diffraction grating with respect to the emission facet to maintain a single optical mode of operation to be maintained.

Description

An Extended Cavity Diode Laser

The present invention relates to a diode laser, to an extended cavity diode laser, to a method of producing laser light, a method of selecting a single longitudinal mode from a laser diode and a method of continuously tuning the wavelength of the laser light whilst maintaining a single longitudinal mode.

Extended cavity diode lasers are also known as external cavity diode lasers.

High resolution spectroscopic investigations require narrow linewidth emitting light sources that are tuneable over as wide a wavelength range as possible to target species of interest with high selectivity and sensitivity. Diode lasers have had a large impact on the field, especially in the mid to near IR (infra red) spectral region and recently also in the visible to near UV (ultra violet) region which allows a multitude of atomic and molecular species to be detected. Applications span fundamental studies of atomic/molecular species, industrial control and sensing applications, to telecommunications and bio-medical applications. Similar narrow linewidth single mode light sources are also required for interferometry, for accurate measurements of vibrations and small displacements.

For spectroscopy of molecular species it is essential that the tuning bandwidth of the excitation laser is as large as possible to allow an unambiguous assignment of individual transitions for species identification, and also to minimise the probability of output discontinuities occurring al the wavelength position one is interested in (such as result as a consequence from a mode- hop). Tuneable diode laser modules are set to replace many much more expensive and bulkier traditional systems to perform high fidelity measurements at improved performance and reduced cost.

On their own, the output of laser diodes is inherently broadband in nature and consists of multiple longitudinal modes. It is known to mount laser diodes in extended cavities to control the laser's emission wavelength and bandwidth, and to narrow down the linewidth of the emission.

One implementation of an extended cavity laser consists of a standard multi- mode Fabry-Perot laser diode mounted in a Littrow-cavity formed by a grating and one facet of the diode crystal itself. The grating acts as a frequency- narrowing device and also acts as the laser's output coupler. Some of the light diffracted by the reflection grating is fed back into the laser diode by retroreflection off the Littrow grating (often the -1 order is used), causing it to operate in a single longitudinal mode, corresponding to the wavelength of the fed back light. Other implementations to achieve this mode discrimination are known, an example being the Littman configuration.

In the aforementioned applications precise and accurate knowledge and control of the output characteristics of the diode laser is essential. One phenomenon, which prevents such accurate knowledge and control, is the phenomenon of mode hopping. Mode hopping can cause sharp discontinuities both in the wavelength and in the intensity output of the device during a wavelength scan. In extended cavity diode lasers, mode hopping can occur when the wavelength of the feedback from the diffraction grating of the external cavity is tuned without appropriate tuning of the effective length of both the internal and external cavities.

The spacing Δv of the modes (in Hz) is determined by the expression: Av = 2 In Where c is the speed of light, / is the cavity length, and n is the index of refraction of the cavity medium.

By adjusting the angle of the diffraction grating the wavelength of the feedback light is tuned, and thus the output wavelength of the extended cavity diode laser system is also tuned. However, a change in wavelength leads to a change in mode spacing and position. Therefore, the external cavity length must also be adjusted with the grating tuning, to keep the external cavity mode in the centre of the grating feedback wavelength profile, and to preserve the mode number. The intrinsic mode structure of the internal diode cavity must also be tuned in synchronicity to match the external cavity modes and the grating feedback profile. Otherwise losses due to competition between the internal and external cavities will occur, through destructive interference between the two electric fields in the two cavities.

Thus all three parameters, grating angle, external cavity length and internal diode cavity length, may be tuned at the same time and at the correct ratios to avoid mode-hops and to preserve single mode operation. Mode-hops will occur when: an external cavity mode, other than the originally active one, overlaps better with the grating feedback wavelength profile; or the internal diode mode and thus receives more gain than the original one; or the internal diode mode shifts too far from the peak of the grating feedback wavelength profile.

Multi mode emission can occur if two external cavity modes experience the same or similar gain.

Thus, in order to allow mode hop free tuning over a wide wavelength range both the cavity length and the angle of the frequency selective device inside the extended cavity have to be varied together with high precision. A known tuning method involves the use of a pivot arm several centimetres long on which the grating is mounted. This arm is rotated around the pivot point by expansion or contraction of a single actuator acting on the arm. A significant disadvantage of this approach is that the position of the arm's pivot point has to be located very precisely in order that rotation about the pivot results in a simultaneous angle change and cavity length change in such a way so as to prevent mode hopping. This requires positioning accurate on a micrometer scale, and a similar degree of mechanical stability of the system. Another disadvantage with this approach is that the cavity length and grating angle cannot be independently adjusted, thus not allowing an optimisation of the overlap between the external mode, the grating feedback wavelength profile and the internal diode mode before wavelength scanning begins. Furthermore, such a design is generally wavelength and cavity length specific. Thus the operating wavelength, the grating period, and the external cavity length all dictate the mechanical cavity design. The design is not flexible in that if a different diode is substituted for the original diode, major efforts, and potentially a complete new set-up, are needed.

In addition, many existing tuning systems require the diode to be antiref lection coated, to avoid mode competition between internal diode modes and extended (external) cavity modes. Internal modes are due to the cavity formed by the two end facets of the diode crystal. Extended cavity modes arise due to reflections between the grating and the nearer face of the diode. To address mode-competition effects, which can cause unstable operation and mode-hops, the output facet of the diode may be antireflection coated before disposing in the extended cavity. The antireflection coating suppresses the oscillating electric field in the internal cavity, and thus reduces the need for tuning the length of the internal diode cavity. However, the process of applying an anti reflective coaling is expensive, requires expert knowledge, and risks damaging the diode irreparably. Hence it would be desirable to avoid it.

It is further known to use an uncoated diode in an extended cavity laser, tuning the effective diode cavity length by varying the injection current applied to the diode, thus avoiding the use of an anti-reflection coated diode. This approach is combined with diffraction grating and/or external cavity length tuning, for example using the known pivot arm technique described above, with its inherent limitations. It is an object of the present invention to at least partly mitigate disadvantages of the prior art.

According to one aspect of the present invention there is provided a diode laser comprising a diffraction grating and a semiconductor laser diode structure having an emission facet arranged to form an extended cavity, the diode laser having mounting means arranged to permit rotational movement of the diffraction grating with respect to the emission facet, characterised in that the mounting means is arranged to permit bulk translation of the diffraction grating with respect to the emission facet to maintain a single optical mode of operation..

Where the diffraction grating forms the output coupler, this leads to a compact device well suited to interferometric applications.

In one embodiment, the emission facet has no antireflection coating.

In one embodiment, the diode structure has a second end face opposed to the emission facet and wherein the second end face is partly reflective

In another embodiment, the diode structure has a second end face opposed to the emission facet and the second end face is wholly reflective.

In one embodiment, the mounting means comprise a plurality of electric actuators, and the diode laser further comprises a control device arranged to operate the actuators to maintain a substantially single wavelength output and mode-hop free operation.

In one embodiment, the diode laser has a current injection circuit for injecting current into the laser structure, and the control device is arranged to vary injection current supplied to maintain said mode-hop free operation to cause the intrinsic mode structure of the internal diode cavity to match the external cavity modes and the grating feedback profile.

In embodiments, the actuators are selected from the group comprising piezo- electric actuators, magnetostrictive devices, electrostrictive devices and stepper motors.

According to a second aspect of the invention there is provided an extended cavity diode laser for producing mode hop free laser light wavelength tuning, comprising; a light emitting diode structure; a wavelength selective element disposed in spaced relationship with said light emitting diode structure thereby to form an extended cavity with said light emitting diode structure for diffracting light from said light emitting device, a plurality of position controllers arranged to vary the position and angle of said diffraction element with respect to said light emitting diode structure, a means for varying injection current flowing through said light emitting diode structure, whereby said variation of position and angle of said diffraction element and of injection current of said light emitting diode structure allows said light emitting device and the extended cavity to allow mode hop free tuning and/or support a single longitudinal optical mode .

According to a further aspect of the invention there is provided an extended cavity diode laser for producing mode hop free laser light wavelength tuning, comprising: a light emitting diode structure; a wavelength selective element disposed in spaced relationship with said light emitting diode structure thereby to form an extended cavity with said light emitting diode structure for diffracting light from said light emitting device, a plurality of position controllers arranged to vary the position of said diffraction element with respect to said light emitting diode structure, and a means for varying the effective cavity length of the material of said light emitting diode structure, whereby said variation of position of said diffraction element and of effective cavity length of said light emitting diode structure allows said light emitting device and the extended cavity to allow mode hope free tuning and/or to support a single longitudinal optical mode .

In embodiments, the diode structure has a gain portion and an optical resonator portion, said resonator portion comprising two opposing optically reflective surfaces.

In embodiments the wavelength selective element comprises a diffraction grating.

In embodiments, each position controller comprises at least one of a piezoelectric device, electromechanical controller, electrostrictive device, or a stepper motor.

In a further aspect, the invention provides a method of producing laser light using a light emitting device and a reflective wavelength selective device in spaced relationship and forming an extended cavity with said light emitting device in order to diffract light from said light emitting device, the method comprising: adjusting the wavelength selective device to vary a frequency of light emitted by said light emitting device and varying a spacing between the frequency selective element and the light emitting device to maintain a single optical mode.

In a still further aspect there is provided a method of continuously tuning the wavelength of laser light while maintaining a single longitudinal mode, the method comprising: providing an injection current through a laser diode whereby said laser diode produces spatially and temporally coherent optical radiation, varying said injection current; illuminating a diffraction element with said optical radiation, and positioning said diffraction element in dependence on the injection current so that a particular order of diffraction provides an output beam and another order of diffraction is fed back into said laser diode. In embodiments, said positioning step comprises Littrow mount positioning.

Embodiments of the present invention address mechanical limitations present in conventional extended cavity diode laser devices and furthermore eliminate the need for anti-reflection coating of the diode crystal facets. Mode-hop free tuning may be achieved over a large wavelength range at a low cost compared to prior technology.

Embodiments of the present invention address these issues to achieve cost efficient and reliable single longitudinal mode tuning by the use of multiple piezoelectric actuator devices to control the extended cavity length and grating position simultaneously with tuning the laser diode current. In embodiments, mode hop free operation is achieved over a tuning range of 110 GHz., as illustrated in Figure 4. This figure shows the transmission through a fixed Fabry-Perot etalon, with a free spectral range of 3.1 GHz, for an extended cavity diode laser wavelength scan. The 37 transmission peaks observed correspond to a total mode hop free wavelength tuning range of 110

Other embodiments use other actuator devices, such as for example magnetostrictive devices, electrostrictive devices or stepper motors, for controlling the positioning and/or orientation of the grating.

In embodiments, the angle and position of the grating can be finely, simultaneously and independently adjusted by the application of voltages on a two or three point actuator mount to which the grating is attached. A change in the grating angle alters the wavelength of the light coupled back into the laser diode. To avoid mode-hops, the grating is axially translated to conserve the mode number of the standing wave set up inside the cavity, as the wavelength of feedback varies. Diode current tuning causes a change in the temperature and of the refractive index in the active region of the diode crystal, thus altering the effective internal cavity length. In combination with movement of the grating this effect can be used to maintain the mode-number of the standing wave both inside and outside the diode laser cavity, and to maintain the overlap between the active mode and the grating feedback wavelength.

An embodiment of the invention will now be described, by way of example with reference to the drawings in which: Figure 1 is a schematic view of an extended cavity diode laser embodying the present invention; Figure 2 is schematic view of the grating mount and the piezoelectric position controllers of the extended cavity diode laser of Figure 1; Figure 3 is a schematic of a typical laser diode structure; Figure 4 shows the 110 GHz wide laser wavelength scan; and Figure 5 shows an alternative arrangement for the wavelength selective element of Figure 1.

Referring first to Figure 1 , an extended cavity diode laser (10) includes a light emitting device (20) having an output facet (22), and a wavelength selective element (30) disposed along a light emitting axis of the crystal so that the optical output facet (22) of the light emitting device (20) and the wavelength selective element (30) form an extended cavity (40). The wavelength selective element (30) is secured to a first mount (70) via two position controllers (36, 37) and the light emitting device (20) is mounted on a second mount (71) fixed relative to the first mount (70). A control device (60) receives a control input at a first terminal (61) and provides respective outputs (62, 63) to a current injection input (21) of the light emitting device (20) and to the control inputs of the position controllers (36, 37). A lens (55) is disposed with its focus at the output facet (22) to collimate light leaving the facet. In this embodiment, the light emitting device (20) is a Fabry-Perot laser diode and the wavelength selective element (30) includes a reflection diffraction grating (32) that serves as an output coupler from which laser light is output without passing again through the laser diode (20).

Again in this embodiment, the extended cavity diode laser (40) includes a reflective element (50), here a mirror, to couple the light output from the diffraction grating (30) to, for example, a metrology or communication system. More than one such mirror may be provided as required. The mirror (50) may be fixedly mounted to the diffraction grating so that movement in an angular direction of the one is compensated by the other, and the light output of the system is in a constant direction.

The wavelength selective element (30) includes a reflective diffraction grating (32) secured to a mount (34) as will more clearly be seen in Figure 2. In this embodiment, the diffraction grating (32) is attached to the mounting member (34) by adhesive. In other embodiments other securing means are used, as appropriate. Continuing to refer to Figure 2, the diffraction grating (32) is generally wedge-shaped in this embodiment. In other embodiments the diffraction grating (132) is generally planar and is disposed on a wedge- shaped mounting member (134) -see Figure 5- and in yet others the diffraction grating and mounting member are generally planar and the mounting member is secured in an orientation so that in the static state light is incident off-normal to the grating.

In this embodiment, the mounting member (34) is generally planar and has opposed ends, and the position controllers (36, 37) are secured near to the ends so that by varying their lengths the mounting member (34) and hence the grating (32) is movable axially with respect to the crystal (20) to allow the whole grating to be bulk translated with respect to the diode facet (22) and angularly with respect to that axis. The purpose of the mounting member (34) is to firmly mount the diffraction grating. The difference in lengths between the position controllers (36, 37) shown in Figure 1 is greatly exaggerated to show the principle of operation.

By suitable inputs to the position controllers (36, 37) the diffraction grating (32) can be moved angularly with respect to the crystal to vary the wavelength to be diffracted back into the laser diode. In this way the desired wavelength is selected and fed back into the laser diode (20). Furthermore, the position controllers (36) also allow the diffraction grating to be translated in an axial direction, thus allowing the number of standing wave nodes in the external cavity and emitted from the crystal to be conserved, i.e. a single mode number to be maintained when the wavelength is changed.

In the described embodiment, the position controllers (36) are piezoelectric crystal actuators. In other embodiments, other position controllers responsive to electrical stimulus may be used, for example a stepper motor or electrostrictive actuator. Combinations are also possible.

In the described embodiment the grating (32) is mounted in a Littrow configuration.

Fig. 3 shows a schematic view of the structure of a typical laser diode (20). The diode has an active region consisting of a gain portion (24) and a resonator portion (23). The gain portion (24) is made up of a semiconductor p- n junction, which emits light in response to an electrical stimulus. The resonator portion (23) comprises the output facet (22) and an opposed facet (28), which is at least partially reflective, allowing the light from the p-n junction to undergo multiple reflections through the gain portion, and thus being amplified. When the losses incurred during the reflections are less than the gain in the gain portion the laser diode will emit spatially and temporally coherent light. The laser diode emission normally consists of several discrete wavelengths, corresponding to several cavity modes emitting laser light. To provide a single or substantially single wavelength output, frequency selective feedback is used, here the reflective grating (32)

Light emitted from the output facet (22) of the laser diode is directed towards the diffraction grating (32) and depending on the angle of incidence of the laser light, light of a known wavelength is fed back into the diode causing the diode to adjust its emission wavelength. The gain bandwidth of the laser diode is broad enough to support several longitudinal modes determined by the effective resonator cavity length of the laser diode. Feedback from the narrower bandwidth external cavity results in single longitudinal mode operation of the laser. The external cavity effectively deselects all other modes other than the mode it can support given the angular and axial position of the grating.

In use, the control device (60) provides control of the piezo-electric crystal actuators and in turn the position of the diffraction grating while synchronously tuning the injection current flowing through the laser diode. Changes in the diode injection current causes a variation in the local temperature and refractive index of the active region of the laser material, which may be for example GaAs, InGaAs, GaN, InGaN, InGaP or AllnGaN. This variation in temperature causes a variation in the effective internal cavity length of the diode laser, which can be used to maintain the number of standing wave nodes inside the diode laser resonator cavity. In this way current tuning of the laser diode can be carried out simultaneously with movement of the diffraction grating, to prevent mode hops from occurring.

In this embodiment, the control device (60) provides three or more electrical control signals to drive the laser diode injection current and the individual piezo-actuators. For laser wavelength scanning the three signals may have the same type of waveform, e.g. triangular, synchronised and in phase. To achieve mode hop free tuning the relative amplitudes and offsets of the signals are adjusted to achieve the correct relations between extended cavity elongation, grating angle tuning, and diode current tuning. For the geometry shown in Figure 1 and for linear wavelength scans triangular waveforms can be used. For other types of wavelength scans other types of functions can be used. The different signals do not have to be of the same type of waveform, but could be individually modified to compensate for non-linearities in actuator and current tuning response.)

It will be appreciated that features of the described embodiment can be varied or interchanged as appropriate, without departing from the invention. For example embodiments may omit the mounting member. The mounting member can also be integral with the diffraction grating, rather than secured to it. Also, other forms of diffraction grating may be used such as transmission gratings. The angular and axial position of the diffraction grating may be controlled using other forms of electromechanical actuators such as stepper motors, electro strictive devices, or magnetostrictive devices. Furthermore, it will be appreciated that any appropriate order of diffraction may be selected, as necessary, for feedback into the laser diode.

It will also be understood that other parameters may affect the effective length of the diode, for example, the bulk temperature of the laser may influence it, and could also be used for actively controlling the internal diode cavity length.

Changes in the temperature of the extended cavity, as caused by changes in the temperature of the surrounding environment, affect the length of the extended cavity. Such changes in cavity length due, for example to changes in the surrounding environment, must be avoided as they can lead to mode hops or drifts in laser wavelength. The temperature of the extended cavity arrangement may be actively controlled just as may be done for the laser diode chip. Active feedback systems or the like may be employed to counteract the effects of the temperature changes by applying an offset level to the piezo-actuator control signals. The current embodiment employs thermo-electric coolers to control the temperature of the extended cavity diode laser, but other types of heaters or coolers can be used for temperature control. I

The nature of the laser diode is not critical to the invention. It would be possible to substitute other types than Fabry-Perot diodes.

An exemplary embodiment of the invention has now been described. The invention is not to be taken as restricted to what has been described but extends instead to the full scope of the appended claims

Claims

CLAIMS:

1. A diode laser comprising a diffraction grating acting as output coupler and a semiconductor laser diode structure having an emission facet arranged to form an extended cavity with the diffraction grating, the diode laser having mounting means arranged to permit rotational movement of the diffraction grating with respect to the emission facet for wavelength selection, characterised in that the mounting means is arranged to permit bulk translation of the diffraction grating with respect to the emission facet to maintain a single optical mode of operation to be maintained..

2. A diode laser according to claim 1 wherein the emission facet has no antireflection coating.

3. A diode laser according to claim 1 or 2 wherein the diode structure has a second end face opposed to the emission facet and wherein the second end face is partly reflective

4. A diode laser according to claim 1 or 2, wherein diode structure has a second end face opposed to the emission facet and wherein the second end face is wholly reflective.

5. A diode laser according to any preceding claim wherein the mounting means comprise a plurality of electric actuators, and further comprising a control device arranged to operate the actuators to maintain a substantially single wavelength output and mode-hop free operation.

6. A diode laser according to claim 5 having a current injection circuit for injecting current into the laser structure, and wherein the control device is arranged to vary injection current supplied to maintain said mode-hop free operation to cause the intrinsic mode structure of the internal diode cavity to match the external cavity modes and the grating feedback profile.

7. A diode laser according to claim 5 or 6, wherein the actuators are selected from the group comprising piezo-electric actuators, magnetostrictive devices, electrostrictive devices and stepper motors.

8. An extended cavity diode laser for producing mode hop free laser light wavelength tuning, comprising; a light emitting diode structure; a wavelength selective element disposed in spaced relationship with said light emitting diode structure thereby to form an extended cavity with said light emitting diode structure for diffracting light from said light emitting device, a plurality of position controllers arranged to vary the position and angle of said diffraction element with respect to said light emitting diode structure, a means for varying injection current flowing through said light emitting diode structure, whereby said variation of position and angle of said diffraction element and of injection current of said light emitting diode structure allows said light emitting device and the extended cavity to allow mode hop free tuning and/or support a single longitudinal optical mode.

9. An extended cavity diode laser for producing mode hop free laser light wavelength tuning, comprising; a light emitting diode structure; a wavelength selective element disposed in spaced relationship with said light emitting diode structure thereby to form an extended cavity with said light emitting diode structure for diffracting light from said light emitting device, a plurality of position controllers arranged to vary the position of said diffraction element with respect to said light emitting diode structure, a means for varying the effective cavity length of the material of said light emitting diode structure, whereby said variation of position of said diffraction element and of effective cavity length of said light emitting diode structure allows said light emitting device and the extended cavity to allow mode hope free tuning and/or to support a single longitudinal optical mode.

10. An extended cavity diode laser as claimed in claim 8 or 9 in which said diode structure has a gain portion and an optical resonator portion, said resonator portion comprising two opposing optically reflective surfaces.

11. An extended cavity diode laser as claimed in claim 8, 9 or 10 in which said wavelength selective element comprises a diffraction grating.

12. An extended cavity diode laser as claimed in any of claims 8 to 11 in which each position controller comprises at least one of a piezo-electric device, electromechanical controller, electrostrictive device, or a stepper motor.

13. A method of producing laser light using a light emitting device and a reflective wavelength selective device in spaced relationship and forming an extended cavity with said light emitting device in order to diffract light from said light emitting device, the method comprising: adjusting the wavelength selective device to vary a frequency of light emitted by said light emitting device and varying a spacing between the frequency selective element and the light-emitting device to maintain a single optical mode.

14. A method of continuously tuning the wavelength of laser light while maintaining a single longitudinal mode, the method comprising: providing an injection current through a laser diode whereby said laser diode produces spatially and temporally coherent optical radiation, varying said injection current; illuminating a diffraction element with said optical radiation, and positioning said diffraction element in dependence on the injection current so that a first order of diffraction provides an output beam and a second order of diffraction is fed back into said laser diode.

15. A method as claimed in claim 14 wherein said positioning step comprises Littrow mount positioning.