WO2010116288A2

WO2010116288A2 - Method and device for detecting coherent radiation

Info

Publication number: WO2010116288A2
Application number: PCT/IB2010/051375
Authority: WO
Inventors: Johannes Baier
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2009-04-06
Filing date: 2010-03-30
Publication date: 2010-10-14
Also published as: WO2010116288A3

Abstract

The present invention relates to a method and device for detecting coherent optical radiation. In the proposed method, an emission wavelength (8) of a laser (5) is tuned or set close to a wavelength (9) of the coherent radiation (3) to be detected. The coherent radiation (3) is coupled into the laser cavity and a resulting periodic modulation in output power of the laser emission of the laser are measured or detected. The proposed method and device are only sensitive to coherent radiation in a narrow wavelength range.

Description

METHOD AND DEVICE FOR DETECTING COHERENT RADIATION

FIELD OF THE INVENTION

The present invention relates to a method and device for detecting coherent optical radiation. The detection or measuring of coherent optical radiation may be necessary in a multiplicity of technical fields, for example in spectroscopy, laser based diagnostics, medical diagnostics or environmental diagnostics.

BACKGROUND OF THE INVENTION

For the detection of optical radiation in most cases either semiconductor or bo Io metric devices are used. In the first case the absorption of photons produces electron hole pairs, which generate a measurable photocurrent. In the second case the heating up due to the absorbed photons induces changes in material parameters, e.g. of the electrical resistivity, which can be measured and yield information on the amount of incident light. Finally, for the detection of light with very low intensity also photomultipliers are used.

These devices and the corresponding methods of detection have in common that the wavelength bandwidth of the detectors is quite broad so that additional measures have to be taken to suppress unwanted wavelength components. The most common method is the use of suited optical filters, like bandpass filters, interference filters, Notch filters etc., which block the unwanted wavelength regions. Furthermore, optical gratings or spectrometers are applied if a very narrow wavelength region shall be selected or if the wavelength of the detected light shall be determined.

All of the above detectors are sensitive to any kind of radiation and do not distinguish between coherent and incoherent radiation sources, so that an incoherent background on a coherent signal can negatively influence the signal to noise level.

An example for coherent radiation to be detected is a coherent Raman signal, which can be generated by stimulated Raman scattering. In this case, for very high excitation intensity of a laser beam exciting a sample volume energy is significantly transferred from the excitation to the so called Stokes frequency. The resulting Stokes radiation obtains the same phase as the exciting laser radiation, resulting in a coherent Stokes signal which is furthermore emitted into a defined direction with low divergence. A second possibility to obtain a coherent signal is to perform the so-called coherent anti-Stokes Raman spectroscopy (CARS). Here two excitation beams are used, the energy difference of which corresponds to the energy spacing of the involved atomic or molecular states. In this case the upper energy level is populated and subsequently probed by a second photon of the excitation beam having the higher energy. The resulting emitted anti-Stokes radiation is again coherently radiated into a specific direction, which is defined by the energy and momentum conservation law of the four photons involved in the process. In order to properly detect the generated signal the frequency components of the excitation beams usually must be filtered before the detection unit and measures have to be taken in order to suppress ambient light.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method and device for detecting coherent radiation, said method and device being only sensitive to coherent radiation in a narrow wavelength region. The object is achieved with the method and device according to claims 1 and

8. Advantageous embodiments of the method and device are subject matter of the dependent claims or will be described in the subsequent portions of the specification.

In the proposed method for detecting coherent radiation, a laser is used as a sensor for the detection of the coherent radiation. The coherent radiation is coupled into the cavity of the laser, the emission wavelength of which being tuned or set to have a wavelength difference to the wavelength of the coherent radiation. The wavelength difference is selected to cause a periodic modulation of output power of laser emission and - in the frequency regime - a beating frequency in the emission of the laser. This periodic modulation of the output power is detected or measured according to the proposed method. In an advantageous embodiment, a SMI laser sensor (SMI: self-mixing interferometry) is used for detecting the coherent radiation. The following detailed explanations relate to such a SMI laser sensor but can also be applied to any other laser which allows self-mixing interferometry and can therefore be used instead of a SMI laser sensor in the proposed method. Laser sensors based on self-mixing interferometry generally provide the possibility of measuring velocities, vibrations and distances and thus cover a broad range of applications. SMI laser sensors make use of the effect, that laser light which is scattered back from a target object and re-enters the laser cavity, interferes with the resonating radiation and thus influences the output properties of the device. When the laser is operated in a suited regime the response to the back coupled light is linear, and the resulting variations in output power or frequency contain traceable information on the movement or the distance of the target object with respect to the sensor. The laser output signal, which contains this information, is typically collected via a photodiode. When using a semiconductor laser, in particular a VCSEL as laser source, the variations in output power or frequency can also be directly detected via the voltage drop over said laser, which exhibits a small component having the same frequency behavior. If the laser is operated with a defined current shape, e.g. a periodic saw tooth or triangular current, the output frequency almost instantaneously follows those current variations due to the simultaneously changed optical resonator length. The resulting difference in frequency between the resonating and the back scattered light can be evaluated in suitable electronics and can be translated back into information about the position of the target object. Information on the velocity of a target object can be obtained even simpler, as it is sufficient to operate the laser sensor with constant current. The velocity information is then contained in the so-called Doppler frequency shift of the back scattered laser light. SMI laser sensors often already contain the optical detector and the control and evaluation electronics to perform the above measurements.

The SMI detection principle uses the interference of two coherent beams within the laser cavity. In the above known applications of such an SMI laser sensor the second beam is a portion of the emitted laser beam reflected by a target object. In case of the present invention, however, the second beam does not origin from the laser emission of the SMI laser sensor. The second beam is a beam of coherent radiation resulting from another light source. This second beam is a coherent signal beam to be detected, which is collected and imaged into the laser cavity of the SMI laser sensor. If this beam of coherent radiation and the laser beam of the SMI laser sensor propagating within the laser cavity are slightly different in frequency, the intensity of the superposition of both beams will be modulated with a beating frequency equal to the frequency difference of both beams. This beating frequency or the corresponding modulation of the output power is detected in the present method with an appropriate optical sensor measuring the laser emission of the SMI laser sensor, or by measuring variations of the voltage drop over the laser. The voltage drop is based on the operation voltage applied for operating the laser and contains an additional modulation due to the interference of the two coherent beams within the laser cavity and the corresponding variation in output power. The detection of such a periodic modulation in output power or a corresponding beating frequency requires the coherent radiation to have a wavelength close to the wavelength of the laser emission of the SMI laser sensor. If the laser emission of the SMI laser sensor is known, the wavelength of the detected coherent radiation can be determined from the beating frequency. From the evaluation of the measured output of the SMI laser sensor, also the intensity of the coherent radiation can be determined.

Since the interference effect used in the proposed method is sensitive only for coherent radiation, which is close to the emission wavelength of the laser, the method allows the selective detection and measurement of coherent radiation without the need of filtering to suppress ambient light or coherent radiation in other wavelength regions.

In the proposed method, the wavelength of the laser emission of the SMI laser sensor has to be tuned to or set close to the wavelength of the coherent radiation to be detected. If this wavelength is not exactly known, the wavelength of the laser emission of the SMI laser sensor may be tuned over a wavelength region in which or close to which the wavelength of the coherent radiation is supposed. The tuning can be performed, for example, by varying the operating current of the SMI laser sensor, which is based on a semiconductor laser, in particular a VCSEL (VCSEL: vertical cavity surface emitting laser). The setting of the laser emission of the SMI laser sensor close to the wavelength of the coherent radiation means that the difference in both wavelengths has to be such that the corresponding difference frequency can be detected or resolved by an appropriate optical sensor or by an appropriate evaluation of the variation of the voltage drop over the laser. Preferably the difference frequency is in the range between 1 MHz and 1 GHz. Known SMI laser sensors comprise a VCSEL, which emits infrared light around 1 μm wavelength with a typical power of a few milliwatts. The operation distance of such a device can typically reach up to several meters. IR VCSEL lasers are quite common in optical communication applications. The laser cavity consists of two stacks of Distributed Bragg Reflectors (DBR), which are epitaxially grown on a suited substrate, and which enclose a gain region made up from several quantum wells. The DBR layers also take over the task of feeding current into the gain region, therefore one is usually n-doped and the other p-doped. One DBR is designed to be highly reflective, typically with a reflectivity of > 99.8 %, while the other one allows a higher degree of out-coupling and thus also feedback to the laser cavity. The big advantage of VCSELs is that due to their surface emitting properties they can be produced and tested on wafer level in large quantities, which opens the possibility of a low cost production process. Furthermore, the output power can be scaled to a certain extent via the area of the emitting surface, and larger output powers can also be achieved by using VCSEL arrays. The working principal of SMI laser sensors is not restricted to surface emitting laser diodes. The SMI effect can e.g. also be harvested in edge emitting devices or solid state lasers.

In an embodiment of the present invention such a SMI laser sensor is used to detect coherent radiation not originating from the SMI laser sensor. Instead of measuring the velocity or distance of a target object, the SMI laser sensor in the present invention is used to detect or measure the frequency of coherent laser radiation. The use of a SMI laser sensor has the advantage that this sensor usually already contains an optical detector, a control unit and a evaluation unit to perform the proposed method. The construction of the SMI laser sensor can be the same as the known SMI laser sensors for measuring distances or velocities. Althohough the use of a SMI laser sensor is advantageus, also any other laser which is sensitve to self-mixing interference can be used to detect the coherent radiation according to the proposed method.

The proposed device including such a SMI laser sensor or other type of laser comprises a control unit configured to tune or set the emission wavelength of the SMI laser sensor or laser close to the wavelength of the coherent radiation to be detected. An optical detector is arranged to measure the output power of the laser emission. Alternatively or in addition, the device comprises a sensing unit for measuring a voltage drop over the laser, the voltage being applied to operate the laser. The proposed device also comprises an evaluation unit configured to detect or determine a beating frequency in a measurement signal of said optical detector or sensing unit.

BRIEF DESCRIPTION OF THE DRAWINGS

The proposed method and device are described in the following by way of example in connection with the accompanying figures without limiting the scope of protection as defined by the claims. The figures show

Fig. 1 an example of a schematic setup of the detection system according to the present invention; and

Fig. 2 a diagram showing the variation of emission laser wavelength of the laser in order to obtain a detectable difference frequency.

DETAILED DESCRIPTION OF EMBODIMENTS

A possible setup of the detection system according to the present invention is schematically indicated in figure 1. A coherent spectroscopic signal is generated by exiting a sample volume 1 with a suited combination of coherent excitation 2, for example by a suited laser system. The generated coherent radiation 3, which may be a Raman signal, is collected via a lens 4 and imaged into the SMI laser sensor 5. This SMI laser sensor 5 comprises a control unit 6 controlling the emission wavelength of this sensor, and an evaluation unit 7 detecting a beating frequency and eventually determining the intensity and/or wavelength of the detected coherent radiation based on the measurement signal of the optical detector or of the measurement of the voltage variation of the operation voltage of the SMI laser sensor 5.

In this example, the SMI laser sensor 5 is based on a surface emitting semiconductor laser (VCSEL) which consists of an electrically pumped gain medium (InGaAs quantum wells embedded in GaAs) embedded between two Distributed Bragg

Reflectors (DBRs), which form the vertical laser cavity. One DBR is highly reflective with a reflectivity of > 99.8 %, while the reflectivity of the second DBR is smaller in order to enable the collection of light imaged from the sample volume 1. One of the DBRs is p-doped and the other n-doped to enable efficient current feeding into the gain region. The diameter of the active area of the surface emitting laser is typically < 10 μm in order to restrict the transverse mode structure to single mode emission.

Via n- and p-DBR electrical contacts a current is fed into the gain region of the SMI laser sensor 5. A photodetector, which is attached to the VCSEL chip, measures the small amount of radiation leaking out of the highly reflective DBR mirror and thus monitors the interference of the two coherent beams within the laser cavity. In an alternative embodiment, the photodetector can also be placed separately from the VCSEL chip, e.g. by separating a small portion of the emitted laser radiation from the main beam with the help of a suited beam splitter. In another alternative embodiment the interference of the two coherent beams is directly monitored via the high frequency variation of the voltage drop over the VCSEL laser.

The presence and magnitude of the coherent radiation 3 is probed via the variation of the wavelength of the SMI laser sensor 5 as sketched in figure 2. This figure shows the tuning of the SMI laser wavelength 8 over a wavelength region including the coherent signal wavelength 9. When scanning the SMI laser wavelength 8 over the coherent signal wavelength 9 a detectable beating at the difference frequency will occur when both frequencies come sufficiently close to each other. Typical difference frequencies which can easily be resolved in a suited electronics, e.g. via fast Fourier transform (FFT), lie in the range between 1 MHz and 1 GHz. This requires an accurate control of the SMI laser frequency via a suited control unit 6. The amplitude of the beating frequency and of the resulting FFT signal, respectively, also contains information on the intensity of the coherent signal radiation, such that this intensity can be determined.

Once the frequency of the SMI laser is known very accurately, e. g. by an upfront calibration of the emission wavelength vs. the driving current, the wavelength of the coherent signal radiation can be determined as well with high accuracy from the beating frequency at a fixed and well-known SMI laser wavelength, making the proposed setup an accurate spectrometer as well.

In order to remain in the linear regime, i. e. the situation, in which a linear superposition of both beams is possible without any distortions or generation of higher order frequency components, the intensity of the coherent signal radiation, which enters the SMI laser sensor 5, is preferably reduced if necessary by suited filters. Another reason for keeping the intensity of the coherent signal radiation below a certain level is to prevent frequency locking of the SMI laser due to stimulated amplification of the in-coupled coherent signal radiation. Generally, in order to be able to obtain a proper difference frequency signal the line width of the SMI laser sensor 5 and the coherent signal radiation 3 have to be compatible, i.e. must have a similar magnitude. The same applies to the coherence length and the phase relaxation times.

While the invention has been illustrated and described in detail in the drawings and forgoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive, the invention is not limited to the disclosed embodiments. The different embodiments described above and in the claims can also be combined. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. For example, the embodiment described above makes use of a VCSEL based SMI laser sensor. It should however been noted that any laser especially also edge emitting semiconductor lasers, which can be varied in frequency and which are sensitive to SMI, can be used. It should furthermore be evident, that the coherent radiation to be detected can be generated by other suited processes besides the Raman effect as well. The proposed method and device are applicable in a multiplicity of technical applications, in which a coherent radiation signal has to be detected or in which the frequency or intensity of such a signal has to be measured. Exemplary applications are in the fields of spectroscopy, of laser based diagnostics, of medical diagnostics or of environmental diagnostics. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope of these claims.

LIST OF REFERENCE SIGNS

1 sample volume

2 coherent excitation

3 coherent radiation

4 lens

5 SMI laser sensor

6 control unit

7 evaluation unit

8 SMI laser wavelength

9 coherent signal wavelength

Claims

CLAIMS:

1. A method of detecting coherent radiation, said method comprising the steps of:

- coupling said coherent radiation (3) into a laser cavity of a laser;

- tuning or setting an emission wavelength (8) of the laser to have a wavelength difference to a wavelength (9) of said coherent radiation (3), the wavelength difference being selected to cause a periodic modulation of output power of laser emission of the laser; and

- detecting or measuring the periodic modulation of output power of the laser emission.

2. The method of claim 1, wherein a self-mixing interference laser sensor (5) is used to detect the coherent radiation (3), said laser being part of the self-mixing interference laser sensor (5).

3. The method of claim 1 or 2, wherein the emission wavelength (8) of the laser is repeatedly tuned over a wavelength region which has said wavelength difference to or includes the wavelength (9) of said coherent radiation (3).

4. The method of claim 1 or 2, wherein the measured modulation of output power is evaluated to determine an intensity of the coherent radiation (3).

5. The method of claim 1 or 2, wherein the measured modulation of output power is evaluated to determine the wavelength (9) of the coherent radiation (3).

6. The method of claim 1 or 2, wherein the laser emission of said laser is measured by an optical detector, in particular a photodiode.

7. The method of claim 1 or 2, wherein said laser is a semiconductor laser operated by applying an operation voltage, and the detection or measurement of the periodic modulation of the output power is performed by sensing a high frequency component of a voltage drop over the semiconductor laser.

8. Device for detecting coherent radiation, said device comprising:

- a laser having a laser cavity;

- a coupling optics (4) arranged to couple said coherent radiation (3) into the laser cavity of the laser; - a control unit (6) configured to tune or set an emission wavelength (8) of the laser to have a wavelength difference to a wavelength (9) of said coherent radiation (3) to be detected, the wavelength difference being selected to cause a periodic modulation of output power of laser emission of the laser;

- at least one of an optical detector arranged to measure the periodic modulation of the output power of the laser emission and a sensing unit for measuring a voltage drop over the laser, the voltage being applied to operate the laser ; and

- an evaluation unit (7) configured to detect or determine a beating frequency in a measurement signal of said optical detector or said sensing unit.

9. The device of claim 8, wherein the laser is a vertical cavity surface emitting laser.