Linear cavity tunable laser

A laser source includes a laser medium having a back facet and a front surface, whereby the laser medium is adapted to emit a laser beam through the front surface into an external cavity defined in length by a cavity end mirror reflecting the laser beam back towards the laser medium. A wavelength tunable filter is arranged between the laser medium and the cavity end mirror adapted for tuning the wavelength of the laser beam. The laser medium, the wavelength tunable filter, and the cavity end mirror are arranged in a spatially linear cavity structure substantially in a line without angular redirection of the laser beam in the cavity between the laser medium and the cavity end mirror. At least one portion of the laser beam within the cavity after passing the wavelength tunable filter and before again passing the laser medium is coupled out as an output beam of the laser source, and the cavity end mirror is provided to be partly transparent for coupling out a first output beam.

BACKGROUND OF THE INVENTION

The present invention relates to wavelength tunable laser sources.

Wavelength tunable laser sources are becoming increasingly important in industry. Typical designs for such tunable laser sources with external cavity are disclosed e.g. in EP-A-921614, EP-A-641052, U.S. Pat. No. 5,263,037, Fuhrmann W et al: “A Continuously Tunable GaAs Diode Laser with an External Resonator”, Applied Physics B. Photophysics and Chemistry, Springer Verlag Heidelberg, vol.B49, no.1, 1 Jul. 1989, p.29–32, XP000034054, or Kawaguchi H et al: “A new class of instabilities in a diode laser with an external cavity”, Applied Physics Letters, American Institute of Physics, New York, vol.45, no., 1 Nov. 1984, p.934–936, XP000706952, ISSN: 0003-6951.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved wavelength tunable laser source preferably fostering miniaturization. The object is solved by the independent claims. Preferred embodiments are shown by the dependent claims.

According to the present invention, a laser source is provided having a spatially linear cavity structure, so that all essential components within the lasing cavity can be arranged in a linear manner (i.e. substantially in a line without requiring angular designs). The laser source comprises a laser medium (e.g. a laser chip) having a back facet and a front surface, whereby the laser medium emits a beam through the front surface into an external cavity defined in length by a cavity end mirror. The laser medium preferably is a semiconductor laser medium. The cavity end mirror reflects the light signal back towards the laser medium, resulting in a resonance behavior in the cavity between the cavity end mirror and the back facet of the laser medium.

A wavelength tunable filter is arranged between the laser medium and the cavity end mirror. The presence of the wavelength tunable filter will allow resonance in the cavity only to such wavelengths or wavelength ranges as defined by its wavelength characteristic. Tuning the wavelength of the wavelength tunable filter thus allows controlling the wavelength(s) of the resonant beam(s) within the cavity.

In a first embodiment of the present invention, the cavity end mirror is provided to be partly transparent, preferably semi-transparent, so that a portion of the beam within the cavity will be coupled out, thus providing a first output of the linear laser cavity. The first output is characterized by high-wavelength purity and low source spontaneous emission (SSE) due to coupling out close to the transmission through the wavelength tunable filter and before being amplified again by the laser medium.

In another preferred embodiment the linear cavity laser further provides a second output derived from providing the back facet of the laser medium to be partly transparent. The second output is characterized by having higher optical power but also higher SSE and lower wavelength purity than the first output. However, by carefully designing the characteristic of the partly transparent cavity end mirror and/or the transparency characteristics of the back facet of the laser medium allows to provide two controlled outputs: one with lower power but higher wavelength purity, and the other with higher power but lower wavelength purity. The characteristics of both the cavity end mirror and the back facet should be carefully adjusted in order to yield in the desired laser characteristics.

In another embodiment of the present invention, a mode hop free lasing can be provided by synchronizing the wavelength selection provided by the wavelength tunable filter with the (effective) optical length of the cavity. The length of the cavity can be modified e.g. in that either one of the cavity end mirror or the laser medium (including the back facet), or both, is linearly moved in the (linear) direction of the laser beam within the cavity. A synchronizing unit preferably controls the wavelength setting of the wavelength tunable filter and the optical path length of the cavity. In case of a laser with continuous wavelength tuning capability, the synchronizing unit preferably controls the mode number to be kept constant over the tuning.

In one embodiment the output of the laser is monitored and applied to the synchronizing unit for synchronizing wavelengths and optical path lengths. In another embodiment, a predefined optical path length will be associated with each wavelength value selectable for the wavelength tunable filter. Carefully adjusting the optical path length to the selected wavelength allows to achieve substantially mode hop free laser output when sweeping through a range of wavelength values.

In a further embodiment, the cavity end mirror is provided with (substantially) hundred percent reflectivity towards the cavity. Instead of coupling out the low SSE first output beam from the cavity end mirror, a beam splitter is provided between the cavity end mirror and the tunable filter for coupling out the first output beam. Alternatively, a beam splitter can be provided between the tunable filter and the laser medium, so that light returning from the tunable filter towards the laser medium will be coupled out (as the first output beam). Although such designs ‘depart’ from the linear arrangement of components, it is to be understood that the linear arrangement is not a ‘must’ for all components but that this linear architecture allows the arrangement of the essential components laser medium, wavelength tunable filter, and cavity end mirror in a space reduced linear manner.

The cavity end mirror can be provided as planar mirror having at least two angular degrees of freedom for adjustment of the mirror. Adjustment can be provided either manually or by means of adequate actuators. The cavity end mirror can also be provided as a curved mirror, preferably together with a focussing optics, which is less sensitive to angular misalignment and easy to align with good cavity stability.

In one embodiment, the cavity end mirror is provided as a three-dimensional retro reflector, preferably as an open corner cube or a solid glass corner cube. In case that the front surface of the cavity end mirror is provided to be fully reflective, all surfaces of the three-dimensional retro reflector or solid glass corner cube are provided with 100% reflectivity. In case that the front surface of the cavity end mirror is provided to be partially reflective, at least one surface of the three-dimensional retro reflector has to be semitransparent.

The invention can be partly embodied or supported by one or more suitable software programs, which can be stored on or otherwise provided by any kind of data carrier, and which might be executed in or by any suitable data processing unit. In particular, control algorithms might be provided by hardware and/or software tools.

DETAILED DESCRIPTION OF THE INVENTION

InFIG. 1, a laser medium10, e.g., a semiconductor active laser gain medium, has a front surface10A and a back facet10B. While the front surface10A is preferably anti-reflection coated, the back facet is preferably provided with a high reflectivity (HR) coating. A laser beam leaving the laser medium10through the front surface10A is passed through a lens20and a wavelength tunable filter30to a cavity end mirror40having a semitransparent front surface40A. A portion of the laser beam is reflected by the front surface40A back towards the wavelength tunable filter30, while another portion is coupled out by the cavity end mirror40as a first output beam50.

The optical path between the front surface40A and the back facet10B provides the cavity of the laser, whereby the optical path between the front surface40A of the cavity end mirror40and the front surface10A of the laser medium10represents an external cavity.

Another portion of the laser beam within the cavity can be coupled out at the back facet10B as a second output60. The second output signal60is preferably launched through a lens70, an isolator80, and a beam splitter90and might be coupled by a lens100into a fiber110. A portion of the second output beam60can be coupled out by the beam splitter90and directed to a detector120for monitoring the second output.

In accordance with the above described coupling out of the second output beam60, the first output signal50is also preferably launched through an isolator130, a beam splitter140, and a lens150to a fiber160. The beam coupled out from the beam splitter140can be directed to a detector170for monitoring the first output beam50.

The wavelength tunable filter30in the cavity provides a wavelength filtering to the laser beam within the cavity, so that only such laser modes can build up that are not filtered out by the wavelength tunable filter30. Controlling the wavelength (range) of the tunable filter30thus allows selecting the desired wavelength or wavelength range for the outputs50and60.

The wavelength tunable filter30preferably is a tunable Fabry Perot filter or an acousto-optic tunable filter.

It is to be understood that the essential laser setup of the embodiment ofFIG. 1only comprises the laser medium10having a partially transparent back facet10B, the wavelength tunable filter30, and the cavity end mirror40having a partially transparent front surface40A. All further components are optional dependent on the specific application and requirements. In particular, the isolators130and80are provided to decouple the cavity from external light signals, in particular light signals reflected from the beams50and60, so that the cavity will not be disturbed from external. This, however, is not essential for the present invention and can be omitted or solved otherwise. The lenses150and100are provided to couple the output beams50and60in the respective fibers160and110, however, other coupling methods and light paths (than the fibers160and110) can be provided accordingly. The beam splitters140and90together with the monitors170and120are optionally provided to monitor the respective output beams50and60. The lens20is preferably provided to generate a collimated beam to achieve high resonator finesse (good quality) at the cavity side, while the lens70is preferably provided to generate a collimated beam to launch the optical power through further components such as isolator and/or beam splitter at the output side.

FromFIG. 1it becomes apparent that all essential cavity components can be arranged in a spatially linear manor, or, in other words, in a line of components without requiring to direct or redirect the light beam in angles (others than 180°) as required for example when employing a Littman or Littrow architecture. This allows to significantly reducing the cavity in size rendering it subject to further miniaturization.

The first output signal50provides a high-purity signal with respect to wavelength purity and signal-to-noise ratio, since it is coupled out directly behind the wavelength tunable filter30and before returning into the laser medium10and being amplified again. The second output signal60, in contrast thereto, provides a high power output at a significantly higher potential output power than the first output signal50but also with higher source spontaneous emission (SSE).

Instead of coupling out both, the first and the second output50and60, the back facet10B can be provided to be fully reflective so that no light is coupled out to the second output60. In that case the first output signal50represents the only output of the linear cavity laser setup.

On the right hand side inFIG. 1, typical spectral shapes are shown for the first and the second output50and60and of the tunable filter30. However, the intensity scale for both outputs is different, with the intensity of the first output being significantly smaller than the intensity of the second output60.

FIG. 2shows another preferred embodiment. Instead of coupling out the first output beam50by means of the front surface40A of the cavity end mirror40, the front surface40A in the embodiment ofFIG. 2is provided to be fully reflective, and a beam splitter200is arranged between the cavity end mirror40and the wavelength tunable filter30. The first output beam50A is derived from the beam splitter200and coupled out in accordance with the embodiment ofFIG. 1. In this embodiment, the essential cavity features are still arranged in a linear manner, and only the output beam50A is redirected from that linear arrangement. The embodiment ofFIG. 1is mechanically easier in case that the cavity end mirror40is provided to be movable (as will be explained below).

In accordance with the above described, the first output signal50A can also be launched through an isolator210, a beam splitter220, and a lens230to a fiber240. The beam coupled out from the beam splitter220can be directed to a detector250for monitoring the first output beam50A.

Alternatively but not shown inFIG. 2, the beam splitter can also be provided between the tunable filter30and the laser medium10(but before the lens20), so that light returning from the tunable filter30towards the laser medium10will be coupled out (as the first output beam50A).

FIG. 3shows a combined approach from the embodiments ofFIGS. 1 and 2. The front surface40A of the cavity end mirror40is provided to be partially transparent emitting the first output beam50as inFIG. 1. The beam splitter200couples out a portion of the cavity laser signal as the output beam50A as inFIG. 2. Thus, a third output can be provided, e.g. for monitoring purposes.

In a preferred embodiment, as indicated in all embodiments ofFIGS. 1–3, the cavity end mirror40is provided to be adjustable in the direction of the cavity length (as indicated by the arrow underneath the cavity end mirror40). This allows to adjust the optical path length of the laser cavity (between the back facet10B and the front surface40A) in order to provide mode hop free laser tuning when modifying the wavelength of the wavelength tunable filter30.

In a preferred embodiment (not shown in Figures), the cavity end mirror40is adjustable in the direction of the cavity length by means of a translation stage. This can be accomplished e.g. by Piezo stack or a spindle driven slider with a guiding mechanism.

In another preferred embodiment, the adjustment of the cavity length by moving the cavity end mirror40as well as the wavelength tuning of the wavelength tunable filter30are controlled and synchronized by a controller (not shown in the Figures).

Instead of, or in combination with, moving the cavity end mirror40, the back facet10B of the laser medium10can be moved, e.g. by moving the laser medium10.

FIG. 4shows a preferred embodiment of the cavity end mirror40. Instead of the cavity end mirror40being a planar mirror, it can be a curved mirror, preferably together with a focussing optics400. The focal length of the focussing optics400, e.g. a lens, should be matched to the mirror curve radius.

In a further embodiment, the cavity end mirror40is a three-dimensional retro reflector (trihedral prism), preferably an open corner cube or a solid glass corner cube e.g. a corner cube prism U43 as provided by Edmund Industrie Optic GmbH. In case of the embodiment ofFIG. 2, wherein the front surface40A is provided to be fully reflective, all surfaces of the three-dimensional retro reflector or solid glass corner cube are provided with 100% reflectivity. In case of the embodiments ofFIG. 1or3, at least one surface of the three-dimensional retro reflector has to be semitransparent.

In case that the cavity end mirror40is provided by a plane mirror, there are at least two angular degrees of freedom for adjustment of the mirror. Adjustment can be provided either manually or by means of adequate actuators. Adjusting collimating lenses in x or y direction could also be applied instead of the two angular degrees of freedom. The collimated beam should preferably hit the end mirror perpendicular.