EP0495874A1

EP0495874A1 - Apparatus for stabilizing the wavelength of a signal-modulated light beam

Info

Publication number: EP0495874A1
Application number: EP19900915543
Authority: EP
Inventors: Hans Göran Evald Martin
Original assignee: Individual
Current assignee: Individual
Priority date: 1989-10-13
Filing date: 1990-10-02
Publication date: 1992-07-29
Also published as: FI921592A; FI921592A0; WO1991006141A1; SE8903363A; SE463952B; SE8903363D0; AU6547090A

Abstract

l'invention concerne un appareil permettant la stabilisation d'une longueur d'onde d'un faisceau lumineux émis (2) généré par un laser à semiconducteur (1) et modulé par un signal (3). Ledit faisceau (2) est détecté par un photorécepteur (4), l'électronique étant connectée audit photorécepteur de sorte qu'une longueur d'onde fixe représentant une transition atomatique selectionnée dans le milieu (M) où se propage ledit faisceau est détectée. Les paramètres fonctionnels (température, courant etc.) du laser (1) sont réglables de sorte que l'émission laser (2) ait une longueur d'onde correspondant, essentiellement, à une longueur d'onde fixe déterminée d'une transition automatique du milieu (M) selectionnée. Un circuit de réglage (8) est conçu de manière à générer les paramètres de signal nécessaires pour que longueur d'onde de tout faisceau lumineux émis (2), dont la déviation de la longueur d'onde fixe est déterminée par la transition atomique, soit réglée à ladite longueur d'onde fixe. Une zone de longueur d'onde pour une étendue de capture est selectionnée en fonction de la longueur d'onde fixe choisie.the invention relates to an apparatus for stabilizing a wavelength of an emitted light beam (2) generated by a semiconductor laser (1) and modulated by a signal (3). Said beam (2) is detected by a photoreceptor (4), the electronics being connected to said photoreceptor so that a fixed wavelength representing a selected atomic transition in the medium (M) where said beam propagates is detected. The functional parameters (temperature, current etc.) of the laser (1) are adjustable so that the laser emission (2) has a wavelength corresponding essentially to a fixed wavelength determined by an automatic transition middle (M) selected. An adjustment circuit (8) is designed so as to generate the signal parameters necessary for the wavelength of any emitted light beam (2), the deviation of the fixed wavelength of which is determined by the atomic transition, is set to said fixed wavelength. A wavelength zone for a capture range is selected according to the fixed wavelength chosen.

Description

Title: Apparatus for Stabilizing the Wavelength of a Signal-modulated Light Beam.

TECHNICAL FIELD

The invention presented here refers to a device or an apparatus for making possible a wavelength or frequency stabilization of a semiconductor laser, where its laser beam in a known manner is modulated by a signal, and where said beam, with superimposed signal, is sensed by a photoreceiver.

BACKGROUND PRIOR ART.

How to stabilize the wavelength and the output power for a semiconductor laser in a precise manner is already known from the publication GB-A-2 163 286, describing the situation where the laser beam, or a part of it, is modulated and allowed to pass through an interferometer or a filter. After detection a feedback signal to the semiconductor laser is generated, using a phase sensitive amplifier and a comparator, to regulate the wavelength of the laser beam towards a fixed wavelength.

This makes a wavelength stabilization up to 10^*9 possible and a regulation of the output power that is better than 10³.

It is known that semiconductor lasers are comparatively inexpensive to purchase but also that they generate an emission, whose wavelength, or light frequency, changes with pressure, temperature and other circumstances, in the range of several percents.

This limits the precision of required data considerably, for example in range finders, gas concentration measurements, etc.

It is obvious that if one and the same semiconductor laser were able, as a function of time, to generate light beams with exactly the same wavelength, it would be able to establish more accurate measuring results.

Practical applications have shown that it is very difficult to make a semiconductor laser generate a lightbeam with one and the same wavelength at different times and to make it work with a fixed wavelength or within a certain narrow wavelength region.

TITUTE SHEET In any case, it is very difficult from one occation or application to another, with different external conditions, to make such a semiconductor laser generate one and the same light beam with a fixed wavelength.

In assessing the basic steps and considerations of the present invention, one should also take into account that much more expensive gas lasers also belong to the previous level of this technology, with the special quality that they at different times are able to generate a light beam with one and the same fixed frequency, or wavelength, in the light beam, essentially independent of external conditions.

The fundamental principle of gas lasers states that the laser frequency is well determined by the transition energy between two possible atomic or molecular quantum states, from here on called "atomic transitions".

It should be mentioned, however, that gas lasers for their operation are based on a discharge, which is, as by nature, inhomogenous and, consequently, gives some variations in frequency and wavelength.

A variable optical distance of the laser cavity is therefore used to adjust for proper fine wavelength. The regulation is usually accomplished by mechanical adjustments, that is the distance between the mirrors is controlled mechanically.

The previous level of technology also includes what is shown and described in US patent 4,730,112, where a device is shown for measuring the absorption of light in a gaseous sample, in particular the oxygen concentration is measured, by using a semiconductor laser.

DESCRIPTION OF THE PRESENT INVENTION

TECHNICAL PROBLEM

Considering the previous level of technology, as expressed by the publications already referred to, and the general considerations discussed concerning the different properties of the semiconductor laser and the gas laser, it must be considered a qualified technical task or problem to offer a simple stabilization of frequency and wavelength of a semi-conductor laser without using accessories like interferometers, wavelength meters, etc.

It must also be considered a qualified technical task or problem to be able to completely eliminate a previously known interferometer, which not only is

STITUTE SHEET comparatively expensive but also shows the inconvenience of giving not enough sharp fringes for a wavelength stabilization.

In addition it must be considered a technical task to create conditions such that a semiconductor will be able to generate a laser beam having one and the same known wavelength at different times.

It must also be considered a technical task and a qualified technical consideration to, depending on a selected fixed known wavelength, choose a proper capture range adapted to assure a locking to one and the same wavelength, at different times, without the risk that the regulation circuitry will lock to another nearby located wavelength.

It must also be considered a technical task to understand the importance of letting a number of parameters attributed to the laser such as actual temperature, laser current, etc. be selected in advance so that the laser with certainty will generate a beam with a wavelength corresponding to the wavelength of one selected "atomic transition" .

It must also be considered a technical task to understand the importance of, depending on a choosen fixed wavelength, choosing a necessary wavelength region for a capturing region, so that the regulation circuitry will not lock on a nearby located fixed wavelength belonging to another atomic transition.

It must also be considered a technical task to adapt the regulation circuitry so that a large capturing range will tend to narrow, when locking is achieved, thereby leaving the region of a nearby located atomic transition.

It must also be considered a technical task to be able to describe an invention, using a semiconductor laser based on the principle of emitting the same fixed wavelength as a preselected atomic transition, assuring that the semiconductor laser will lock at said wavelength every time it is used and hereby offer laser emission having an unambiguously determined and stable frequency and wavelength with seven to nine digits of accuracy.

It must also be considered a qualified technical task to achieve a wavelength stabilization of a semiconductor laser modulated by a signal, for instance somewhere between 500 Hz and 20 kHz, whereafter said light beam is sensed by a photo-receiver with circuitry arranged for sensing a preselected atomic transition in the medium where the laser light has passed and, in addition, in a known manner to show electronic circuitries that are arranged for affecting the semiconductor laser current and/or the actual semiconductor laser temperature such that any tendency of change in wavelength will be compensated for by a regulation of the injected laser current and/or laser temperature.

SUBSTITUTE SHEET It must also be considered a technical task to create conditions at different times of use so that a semiconductor laser directly will emit one particular wavelength, corresponding to one out of several available atomic transitions, for instance those of oxygen, and to lock and regulate the emitted wavelength towards this selected fixed wavelength.

It must be considered a technichal problem to be able to realize the importance of letting the signal of the said receiver in a known manner be the subject of a signal treatment, aimed to offer a well defined regulation region with a clear negative and a clear positive value on each side of said selected fixed wavelength, and to realize that a regulation of the amplitude of the modulated signal should be variable so that with increasing value an increased regulation, or capture, region will be achieved and with less value of the amplitude the defined capturing region will be decreased with steeper regulation parameters for the realization of an automatic search against and locking to a selected fixed wavelength corresponding to one preselected chosen atomic transition.

Consequently, it has to be considered a technical problem to realize the importance of, during the start up sequence for such a semiconductor laser, varying the amplitude of the modulated signal in such a way that it decreases continuously from a high level to a normal level of modulation, thereby approaching a well defined and narrow regulation region, in which proper wavelength stabilization is acting.

It must also be considered a technical problem to realize the importance of connecting a circuitry to the photo receiver for automatic sensitivity and/or automatic gain control, thereby achieving necessary conditions for said wavelength stabilization, irrespective of external conditions .

Since the beam of a semiconductor laser, arranged for wavelength stabilization by using its own photosignal, contains information about at least one atomic transition in the medium where the light beam passes and when several such fixed wavelengths corresponding to different atomic transitions can not be excluded for one and the same medium such as air, it has to be considered a technical problem that for the adjustment of one fundamental wavelength among several possible atomic transitions to realize that the temperature for this laser has to be selected within a narrow temperature region and the current through the semiconductor laser has to be adjusted within a narrow region, which in practise means that every semiconductor laser used has to be precalibrated such that the wavelength of the emitted laser beam is determined as a function of temperature and laser current.

SUBSTITUTE SHEET It also has to be considered a technical consideration, including a qualified technical problem, that for measuring a concentration of a gas in a medium, transmitted by the laser beam, using a semiconductor laser, one only need to evaluate the value of the amplitude in the photodetector, receiving said laser beam with the fixed wavelength, where this value of the amplitude is comparable with the amplitude value of another signal for the same wavelength acting as a calibration of the same atomic transition in a well defined known concentration.

In addition, it has to be considered a qualified technical achievment to be able to offer a low cost range finder using a semiconductor laser locked onto a fixed wavelength corresponding to a selected atomic transition in a medium where the laser beam propagates and where the distance measuring can be done with an accuracy of up to 10 -⁹.

Finally, it is a technical problem to realize that a wavelength stabilized semiconductor laser, with its light beam modulated by simple means, should be able to cooperate with a second semiconductor laser, which thereby can be forced towards a fixed wavelength whithout modulation of its own light beam.

THE PROPOSED SOLUTION

To be able to solve one or several of the technical tasks or problems mentioned, the present invention starts by offering a device or an apparatus making possible a wavelength stabilization of a semiconductor laser where its laser beam is modulated by a signal, for instance 2 kHz, whereafter said signal is sensed by a photo-receiver.

In this photo-receiver the ciriuitry should be arranged in such a way that it can sense the atomic transitions in the medium where the light beam propagates.

According to the present invention, it is now proposed that the laser is attributed parameters (temperature, current, etc.) that are adjustable so that the laser emits a light beam with a wavelength corresponding to, or dominantly corresponding to, the wavelength of one selected atomic transition. A regulation circuitry is arranged such that if the emitted light beam deviates from the wavelength of the atomic transition it will generate error signals for the attributed laser parameters such that the wavelength is forced towards the fixed wavelength selected. The necessary wavelength region for a capturing range must be chosen depending on which fixed wavelength is chosen, taking into account necessary information regarding other nearby located optical resonances.

As a proposed apparatus or device, within the frame of the present invention, it is proposed that said regulation circuitry should be arranged in such a way that

E SHEET from the detected intensity variation of the light beam a wavelength changing signal is generated, adapted to give an electrical influence of the semiconductor laser in such a way that it changes its emission wavelength towards said fixed wavelength.

The regulation circuitry is arranged such that it affects the semiconductor injection current and/or semiconductor laser temperature such that every tendency of wavelength change will be compensated for by an appropriate variation in the laser current and/or actual temperature.

As an additional proposed device, within the frame of the present invention idea, it is proposed that said modulating signal should be the subject of an adjustment in order to offer a well defined regulation region, or capturing region, resulting in a pronounced negative and a pronounced positive feedback signal value, respectively, on each side of said fixed chosen wavelength.

It is further proposed that the modulation signal amplitude is variable so that with an increased value the defined capturing range increases, particularly during a start up sequence, whereafter the amplitude of the signal should be continuously decreased from this high level down to a normal level, in order to approach a well defined narrow regulation region.

It is further proposed that, to the light receiver there is attached a circuitry for automatic sensitivity adjustment and/or automatic gain control.

It is further proposed that for the pre-adjustment of the wavelength to one chosen atomic transition, out of many possible, the temperature of the laser is chosen to a value which at a predetermined laser current will make the laser emit at a wavelength which in any case will be close to the wavelength of said chosen atomic transition.

Further a method is proposed, using a wavelength stabilized semiconductor laser of the sort described above, of measuring the concentration of a gas in a beam propagating medium, where the value of this concentration is evaluated from the value of the received light intensity at a chosen fixed wavelength. This is trivial in the case where another signal is calibrated for the same atomic transition at a certain established concentration.

This measure of the gas concentration is made by a sequential measurement.

A somewhat more complex device to establish the momentary value of a gas concentration, uses two laser beams in a parallel measurement, one wavelength stabilized beam in a reference cell containing a know gas concentration and one

BSTITUTE SHEET beam through the concentration of gas to be evaluated. In this way it is possible to evaluate the unknown concentration using a continuous calibration.

Another possibility is to have the semiconductor laser locked to an atomic transition in a gas, for instance oxygen, and having another beam from the same laser passing through a solution, in which the momentary concentration (of oxygen) can be evaluated.

The latter situation holds, of course, only if the fluid offers a transmission of the laser wavelength that depends on the concentration.

The invention also offers the possibility of measuring a fluid concentration by parallel measurements by simply determining the amplitude of one of the photo- signals, when the amplitude value of another signal is calibrated for the same atomic transition in a certain established concentration in the same fluid.

At range findings the emitted wavelength should be locked to a chosen atomic transition for a gas found in air.

Finally, for achieving an extremely good wavelength stabilization, a device is proposed where the light from one wavelength-locked laser, modulated around an atomic transition as already discussed, is interacting with that of a second non-modulated laser, having the same centre wavelength as the first one, thereby producing a heterodyne signal on a second fast response photo-receiver, which is used to produce an error signal for the centre wavelength of said second non- modulated laser, which then indirectly becomes locked to said atomic transition.

BENEFITS or ADVANTAGES

The benefits linked to a device, functioning in accordance to the principles of the present invention, are the creation of possibilities of, by simple means, using a semiconductor laser and making its wavelength stable at a wavelength determined by an atomic transition in a gas, present in the beam propagating medium, and making the semiconductor laser lock at said wavelength at any time, by choosing a proper capturing range and regulation region selected in such a way that the laser wavelength will not lock onto an adjacent atomic transition, and consequently a "wrong" wavelength.

What mainly characterizes an apparatus or a device, using the principles of the present invention, is presented in the characterizing part of Claim 1. BRIEF DESCRIPTION OF THE FIGURES

The realization of a device, showing the significant features related to the principles of the present invention, will here be described more in detail with reference to the drawings where;

Figure 1 shows, in a very simplified scheme, the principles of a device for making possible a wavelength stabilization of the light beam emitted by a semiconductor laser, to a chosen fixed wavelength determined by an atomic transition,

Figure 2 shows a circuit for automatic sensitivity adjustment and automatic gain control, connected to a photo-receiver,

Figure 3a shows the transmittance properties of the earth's atmosphere at low spectral resolution as a function of wavelength of light propagating perpendicular to the surface of the earth,

Figure 3b shows a part of Figure 3a magnified, in the region of oxygen absorption, around 0.76 pm wavelength,

Figure 3c shows a part of Figure 3b magnified, showing oxygen absorption wavelengths at high resolution,

Figure 4a shows a signal obtained from an analogue electro-optic derivation of the wavelength dependent signal in figure 3c, suitable for regulation of the semiconductor laser towards a fixed wavelength "λ_L",

Figure 4b shows how the wavelength dependent signal in Figure 4a is changed when the applied modulation amplitude is increased compared to the case in Figure 4a,

Figure 5 shows a normalized detector signal suitable for wavelength stabilization without the need to modulate the laser,

Figure 6 shows a device for wavelength stabilizing a semiconductor laser for the purpose of using it for measuring the concentration of gas in an unknown sample, relative to a known reference sample, to which by absorption the wavelength of the laser is locked, where the dotted parts of the Figure are meant to illustrate a possible way for a continuous correction of possible time

SUBSTITUTE SHEET varying optical losses, using a second laser, not shown in the figure, having a wavelength differing from any atomic absorption wavelength, and

Figure 7 shows a device where one semiconductor laser is stabilized in accordance with the principles of the invention, and a second laser is made, by heterodyne technique in addition to the principles of the present invention, to emit the same central wavelength as the first one, but without any modulation superimposed.

DESCRIPTION OF THE PROPOSED EMBOD I MENTS OF THE

INVENTION

Referring to Figure 1, a device is shown, using a block diagram, for making possible a frequency, or wavelength, stabilization of a beam 2, emitted by a semiconductor laser 1, where said beam is modulated by a LF-signal generated by the electronics 3. This modulation can vary and is here chosen to 2 kHz.

The light beam 2 passes the medium "M" and is sensed in a photo-receiver 4, which by a wire 5 is connected to an automatic sensitivity adjustment and an automatic gain control 6, with its block diagram, shown in Figure 2.

Figure 1 shows that the circuit 6 is connected by a wire 8 ' with a phase sensitive amplifier 8, arranged for demodulating the electronic signals from circuit 6 and, as a result, generating a regulating signal to the semiconductor laser 1 so that this will increase or decrease the wavelength of the emitted beam 2.

This regulating signal is consequently arranged for affecting the semiconductor laser 1 such that it is automatically wavelength stabilized towards a fixed wavelength.

A temperature regulator 7 keeps the laser at a certain temperature. Alternatively, the temperature regulation can be accomplished using a photo- receiver, monitoring the laser intensity - too high intensity when the wavelength is locked means an increase in the temperature, and vice versa.

Referring to Figure 2, a photo-receiver 4 setup is shown with an automatic sensitivity adjustment 6a, a buffer amplifier 6b, and an automatic gain control 6c. Since these different parts are previously known as such, no detailed schematics are given on these subjects.

The electronics 6c is connected to the amplifier and demodulator circuit 8, using the wire 8'.

TITUTE SHEET In Figure 2 different functions of the electrical signals transmitted in the wire 8' are also shown.

The upper photo signal 10 illustrates the constant amplitude, controlled by the electronics 6, and shows two significant absorption dips 10a and 10b, due to one atomic resonance transition. The signal is conditioned by the electronics 8, by familiar synchronous or rectifying technique, which in this case will generate a positive error signal, which will regulate the operating parameters for the semiconductor laser 1 to increase its emitting wavelength.

The middle signal 11 indicates that absorption dips 11a and lib occur in such a way that the electronics 8 will, in this case, generate a negative error signal with a decreasing wavelength change as a result.

The lower signal function 12 indicates an error free situation at a fix chosen wavelength, in which case the absorption dips 12a and 12b are in the correct symmetric sequence.

In Figure 3a the transmittance properties of the earth's atmosphere are shown at low spectral resolution for light propagating perpendicular towards the earth's surface. The graph represents the wavelength dependent intensity variation seen by the photo-receiver 4 when a laser beam 2 propagates through the air.

For a wavelength stabilization, in accordance with the present invention, one can here choose one, among a large number of distinct absorption dips, since they all represent atomic transitions.

In the forthcoming discussion, this will be illustrated by one fix wavelength (approximately 0.7635 um) representing one particular transition in oxygen "O₂".

The principles of the present invention cannot be illustrated using the Figure 3a, and not even by the enlargement in Figure 3b, but need a further enlargement of the oxygen absorption spectrum 3b, us shown in Figure 3c.

In figure 3c one can see that the wavelength dependent photo signal 13, representing the absorption spectrum at high resolution, shows a pronounced signal dip 13a, which in this example has been chosen to be the fix wavelength standard "λ_L".

This dip 13a, has a line width of about 0.000006 μm at atmospheric pressure and is symmetrical around the fix wavelength "λ_L".

SUBSTITUTE SHEET Of importance, for the invention, is to know the appearence of adjacent signal dips 13b and the magnitude of the wavelength difference 13d. With this knowledge, it is possible to make the regulation electronics selective enough to guarantee that the wavelength cannot be locked onto the "wrong" signal dip 13b.

In this particular example, when the laser operating conditions are such that the emission wavelength is close to "λ_L", a decrease of the laser temperature of 1 °C at constant laser current "i", will shift the wavelength so that a locking to the dip 13b may take place.

At a constant laser temperature T, a current deviation of about 15 mA may also cause the risk of trapping the wavelength in the wrong dip 13b.

Consequently, if the temperature T is preset, with a reproducibility by at least 0.5 °C and the current "i" by at least 7 mA, to the correct values giving the emission "λ_L", then the electronics can be made to regulate the wavelength towards the predetermined fix wavelength.

Figure 4a shows the centre wavelength dependent error signal, for the situation shown in Figure 3c, caused by the electro-optic derivation obtained from the laser wavelength modulation. The signal demodulation and low pass filtering take place in the electronics 8, making the signal 4a suitable for regulating the semiconductor laser towards the fix wavelength "λ_L".

The signal 14 shows a positive region 14a and a negative region 14b, which together form the "capturing" range "A" in between.

It is evident that a change of the wavelength in the light beam 2 causes a change in the signal voltage, which can be used as a feedback signal to regulate the laser towards the fix wavelength "λ_L".

The magnitude of the capture range "A" depends on the amplitude of the modulating signal and, consequently, can be reduced towards the lower limit, which equals the oxygen linewidth.

The correct wavelength locking, however, can be facilitated by making the capture range "A" wider during a start up sequence, by a temporary increase of the laser modulation amplitude. This situation is illustrated in Figure 4b, which also shows the assymetries aroused in the error signals in this case, caused by the interferences between the enlarged regions 14a on one side, and 14e, in particular, on the other side, originating from the presence of dip 13b in Figure 3c.

E SHEET An alternative to the startup sequence just described, is to give the predetermined laser operation parameters starting values that produce a startup wavelength that is on the "safe", in this case, long wavelength side of "λ_L". A constant low offset, added to the regulating error signal, will in this^" example tend to reduce the laser wavelength until it reaches the capture range "A" and, consequently, lock on "λ_L" without the risk of locking to the "wrong" dip 13b.

For a more complete understanding of some parts of the diagram in Figure 1, the content of the publication GB-A-2 163286 is referred to.

As a summary, the semiconductor laser used will have to be calibrated, or so investigated, that one knows at what temperature and what laser current, or other indirect measures of these quantities, a certain emission wavelength occurs, so that a good choice of these operational parameters can be made, in order to force the laser towards a certain chosen wavelength.

Furthermore, some knowledge is necessary about possible adjacent absorption dips, so that the proper capture range "A" and the modulating signal amplitude, is chosen, and a small error signal offset may be added for the "search" of the region "A" upwards or downwards in wavelength, so that locking is achieved to one preselected fix wavelength, among several possible ones.

The electronic principle shown in Figure 2 solves the problem of making the following signal processing independent of time, in spite of time dependent factors, like dirt accumulation, dealignment of the optics, vibrations and other possible disturbances. Furthermore, it offers a possibility for subtraction the pure AM component in the photo-receiver signal, the "carrier wave", by using, for instance, the common laser diode internal emission intensity monitor in a duplicate amplifier scheme, thereby making it possible to amplify the pure effect of atomic transitions to a larger extent, than otherwise possible, especially in cases when this absorption is very weak compared to the pure amplitude modulation.

A wavelength stabilized semiconductor laser, according to the principles already discussed, is very well suited for a sequential measurement of actual gas concentration, simply by evaluating the recieved signal amplitude, in particular the analogue second derivative generated from the signal function 3c as the laser wavelength is modulated. For the purpose of calibration, it is convenient to take advantage of the amplitude value of a second signal, obtained for the same atomic transition but at a known concentration.

SUBSTITUTE SHEET For the linear absorption range, it is known that a very linear relation exists between the gas concentration and the second derivative of the absorption signal.

Suppose the medium "M" is natural air and the wavelength stabilization is made using a transition in oxygen, then the absorption amplitude 13a' of the signal 13a shown in Figure 3c, can be calibrated to 21,6% oxygen.

A wavelength stabilized semiconductor laser, as described above, can also be used for measuring actual concentration in a fluid using a parallel measurement and simply evaluating the amplitude of a signal. Also here, it is suggested that, for the purpose of calibration, the amplitude of a second signal at a certain wavelength and a known concentration is used.

The present invention thus offers a possibility of wavelength stabilization, at the accuracy previously discussed, and is based on a feedback of the processed signal from an atomic/molecular transition. By a slight wavelength modulation of the laser, in combination with phase sensitive detection of a wavelength dependent signal (i.e. laser absorption, laser induced emission, opto-galvanic effect, or any other laser spectroscopic method of detection), a derivation of the latter signal can be obtained (odd harmonics for regulation purposes), which after proper amplification can serve as the error signal for the regulation circuit. By this procedure the laser wavelength is locked to the central wavelength of the atomic transition.

Also without any modulation the laser wavelength can be locked to said atomic reference, but in this case not to the central wavelength but to the slopes of the absorption line 13a. This is shown for the inverted signal in Figure 5.

This locking position is defined by a user set reference signal level (ref). If this dispersive signal by nature is stable in the amplitude, or otherwise is normalized in a proper manner with respect to the signal affecting factors, then this method of stabilization may also be good, giving a reproducible wavelength that is known or can be calibrated.

A combination of the proposed methods for wavelength stabilization can under some circumstances provide new advantages. For example, a non-modulated highly stable semiconductor laser can be locked with an even higher accuracy to a temperature stabilized long high-finesse interferometer, whose cavity modes are actively coupled to an atomic transition using, for instance, a second wavelength stabilized laser in the spirit of the present invention. In this way stabilized laser wavelengths can be realized that do not necessarily have to coincide with the atomic transition wavelengths available, but can differ from these by some multiple of the interferometer free spectral range.

EET The formulation of the claims are intended also to include extensive interpretation.

In addition, a non-modulated laser, with a stable long coherence length, can be locked to the centre wavelength of a modulating wavelength stabilized laser, discussed previously, through heterodyne technique, creating an even higher wavelength accuracy coupled to an atomic transition.

Also, within the frame of the present invention, is the possibility of using one of the resonance wavelengths found in natural air. Oxygen (0.76 μm), water vapour (0.81 μm, 0.92 μm, 1.1 μm, 1.4 μm, 1.9 μm), and carbon dioxide (1.4 μm, 2.0 μm, 2.7 μm) are available everywhere at the surface of the earth without needing to include any extra gas cell in the device (the wavelengths in parantheses show some examples on spectral regions for these gases, where absorption wavelengths are found that can be used for the realization of present invention). A reflexion of some percent of the laser beam from some external optical component, may give information enough for the system to lock its wavelength at a pre-determined absorption line in the air.

In order to reach predetermined and known reference wavelengths, the laser temperature can, for example, be regulated to a predetermined temperature chosen such that the resulting laser wavelength is approximately correct, good enough for the electronics to be within the capture range, or automatically drift towards the capture range, for the selected absorption line. The laser modulation amplitude can during this startup sequence be enlarged, in order to temporarily increase the locking dynamics.

Figure 6 describes a device in the spirit of the invention. A low-cost commercially available GaAlAs-semiconductor laser 30 (Mitsubishi ML4405) emits a laser beam through a pathlength 31-32-33 to a photo-receiver 34. The wavelength is in the 0.76 μm spectral region where oxygen has absorption lines with absorption strengths of the order of two percent per metre air. No other species than oxygen has significant absorptions in this chosen spectral region. The laser 30 has an internal photo-receiver monitoring the intensity of the emitted laser radiation. If this tends to be too high, a transistor 38 lowers the regulation voltage to the laser current regulator 39, in order to prevent the laser intensity from increasing additionally.

During startup the laser will partly be heated fast, by the laser current, and partly slowly, by the resistive heating 40.

Heating results in an increase of the laser wavelength. The laser is wavelength modulated by the oscillator 34, through a small modulation of the laser current.

SUBSTITUTE SHEET After signal conditioning 35 (equivalent to the electronics 6 in Figures 1 and 2) of the detector 34 a.c. components, the phase sensitive (odd harmonics, preferably three times the modulation frequency) demodulator 36 output is biased by a constant offset in adder 37. This offset is high enough to make the laser regulator 39 drive the laser as high as to execute the laser light limit function. The only event that can make the light level decrease, and the transistor 38 open up the ground connection, is that a spectral absorption line will be scanned through as the laser wavelength is increased. When this happens, the amplified negative going signal of the demodulator output 36 will affect the laser regulator 39 so that the laser intensity will decrease and, consequently, the transistor 38 will disconnect the ground. The driving of the laser has now turned from a light regulated mode into a current regulated mode, regulated by the derivative signal of the spectral absorption line, that is a wavelength stabilized situation. The laser wavelength is trapped to that of the atomic transition. The light limiting signal that affects the transistor 38 also affects the transistor 41, which in turn regulates the laser ambient temperature through the resistive heating 40. In this way, the heating of the laser will stop once the laser wavelength have been trapped by an atomic absorption, and the long term stabilization will be assured through this temperature regulation, which keeps the laser intensity at a level that optimizes the dynamics of the wavelength capture range by fast laser current regulation.

The device just described can be expanded to give a measure of the concentration of the molecules/atoms to which transition the laser wavelength is locked. Let the pathlength 31-32-33 constitute a reference cell containing said gas at a known concentration. Let an equally long pathlength 31-42-43 constitute a measuring cell containing a sample gas with an unknown concentration of the reference medium to be determined.

As a measure of the two different absorption strengths is the first harmonic overtone (or any even multiple of this) of the laser modulation frequency, resulting in symmetric line profiles with maximum amplitudes at the stabilized wavelength. The signal-to-noise ratio of the absorption signal is generally better for the harmonics than for the fundamental frequency, because of possible noise in the amplitude modulation. The selected overtone is filtered by the demodulator circuits 44 and 45. A differential amplifier 46 will make the detection more sensitive and reduce unwanted effects of noise in the laser in addition to supressing the pure laser amplitude modulation (the carrier wave). An analogue divider 47 finally computes in real time the percentage deviation between the two concentrations in the form (A-B)/A which is presented on the display 48.

In order to minimize different impacts of accumulating dirt, and other time dependencies of the two different gas cells, one can superimpose the two optical

SUBSTITUTE SHEET pathlengths with the light from another laser that is not in resonance with any atoms or molecules in the gases. This laser is modulated at another frequency 53, different from that of the wavelength stabilized laser, and also illuminates the photo-receivers 33 and 43. After phase-sensitive detection and amplification at this different fundamental frequency, and further low pass filtering, the influence of the pure optical losses in the system, measured using this second laser wavelength, can be traced in the parallel electronics and used to normalize the gas measuring signals according to the formula (A1-B1)/A1 ^■ A2/(A2-B2).

It is worth noting, in the method for measuring a gas concentration just described, the value of the wavelength is of no importance at all. Consequently, the laser is allowed to lock at the first absorption line found for the correct gas. For range finding applications, however, the previously described startup sequence will have to be used, in order to assure one and the same known laser wavelength in every situation, irrespective of different environmental conditions. In a practical device, it is also suggested that if any disturbance make the laser loose track of the selected atomic transition, then a reset signal should automatically provide for a new startup sequence with a new automatic search for said transition.

In the startup sequence, it is therefore a good solution to increase the modulation amplitude considerably in order to increase the capturing range and, hence, reduce the requirement of an exact laser temperature. As soon as locking is achieved, the modulation amplitude shall be decreased to a small value comparable to the value of the spectral linewidth, or even smaller, in cases when wavelength accuracy is of great importance. This facility is easily accomplished, for instance, using a semi-grounded transistor connected to the centre leads of a voltage divider, scaling down the modulation amplitude with a time constant given by a RC network connected to the base of said transistor.

It should also be noted, that a gas at low pressure gives a more narrow linewidth and, consequently, a higher wavelength precision, compared to the same gas under atmospheric pressure.

Figure 7 shows an apparatus where one semiconductor laser 1 is stabilized according to the principles of the invention, and this is made to interact with a second laser 1 ' by heterodyne technique, for the purpose of making a long term wavelength stabilized laser beam at a known wavelength and without any modulation superimposed, that is a beam with a very long coherence length.

This is accomplished by a coherent superposition 22 of one part of the beam 2, emitted by laser 1, with one part of beam 2', emitted by laser 1 '.

SUBSTITUTE SHEET The beat frequency of the optical wave 22 is registered by the fast photo-receiver 4', and processed by the wide bandwidth electronics 6', which generates the necessary error signal to regulate the laser 1 ' to emit a wavelength of beam 2' that equals the central atomic wavelength, without any modulation of beam 2'.

The invention is, of course, not limited to the discussions and examples of apparatus given here, but can be modified within the frame of the patent claims, here to follow, illustrating the idea of the invention.

UTE SHEET

Claims

1. Apparatus for stabilizing the wavelength of a signal-modulated light beam generated by and emitted from a semiconductor laser, said beam being sensed in a photo-receiver connected to electronics which function to sense the fixed wavelengths of atomic transitions in the medium through which the light beam passes, c h a r a c t e r i z e d in that the operational parameters of the laser (such as temperature, injection current, etc.) are chosen so as to generate a laser beam with a wavelength corresponding to, or substan¬ tially to, a predetermined fixed wavelength of a par¬ ticular selected atomic transition; in that said ap- paratus includes regulating circuitry which functions to correct any generated light beam whose wavelength deviates from the particular fixed wavelength deter¬ mined by said atomic transition, by generating the laser operational parameters necessary to adjust the wavelength of the emitted laser beam towards said fixed wavelength; and in that the requisite wavelength range for a capturing range is chosen with respect to the chosen fixed wavelength.

2. Apparatus according to Claim 1, c h a r a c ¬ t e r i z e d in that the regulating circuitry is constructed to generate a wavelength-changing signal from a detected intensity variation of a light beam, by changing the operational conditions of the semiconduc- tor laser such as to change the wavelength of the emitted laser beam towards said fixed wavelength.

3. Apparatus according to Claim 1 or Claim 2, c a r a c t e r i z e d in that said regulating circuitry is constructed to so influence the

SUBSTITUTESHEET semiconductor laser injection current and/or the semi¬ conductor laser temperature that every tendency towards a change in wavelength will be compensated for by an adjustment of the laser injection current and/or of the temperature.

4. Apparatus according to Claim 1, c h a r a c ¬ t e r i z e d in that the modulated signal is sub¬ jected to adjustment in order to provide a well-defined spectral regulation region or spectral capturing range having one pronounced negative value and one pronounced positive value on each respective side of the fixed wavelength.

5. Apparatus according to Claim 4, c h a r a c ¬ t e r i z e d in that the amplitude of the modulating signal can be adjusted in a manner to increase the value of the defined regulation region or capturing range.

6. Apparatus according to Claim 1, Claim 4 or Claim 5, c h a r a c t e r i z e d in that during a start-up sequence, the amplitude of the modulating signal decreases from a high level down to a normal level, so as to approach a well-defined narrow regula¬ tion region.

7. Apparatus according to Claim 1, c h a r a c ¬ t e r i z e d in that circuitry for automatic sen- sitivity adjustment and/or automatic gain control is connected to the photo receiver.

8. Apparatus according to Claim 1, for the purpose of presetting the wavelength to an atomic transition selected from a number of possible atomic transitions, the laser temperature is selected at a value which lies at least close to said fixed wavelength at a predeter¬ mined laser current.

9. Apparatus according to Claim 1, c h a r a c ¬ t e r i z e d in that the amplitude value of the fixed wavelength received by the photo-receiver is evaluated in order to determine the gas concentration of a medium through which the light beam passes, wherein this amplitude value can be compared with the amplitude value of another signal for the same wavelength cali¬ brated for the same atomic transition in a given established concentration.

10. Apparatus according to Claim l, the amplitude value of the signal is evaluated for the purpose of measuring fluid concentration, wherein the amplitude value of another signal is calibrated for a given atomic transition in a given established fluid concentration.

11. Apparatus according to Claim 1, c h a r a c ¬ t e r i z e d in that the emitted wavelength is locked to an atomic transition of a gas present in air, for the purpose of measuring a distance or length.

12. Apparatus according to Claim 1, c h a r a c ¬ t e r i z e d in that the thus modulated light beam is made to interact coherently, heterodyne, with another light beam emitted by a second semiconductor, in order to regulate said second beam of light towards a chosen fixed wavelength.

SUBSTITUTESHEET