WO2019193587A1

WO2019193587A1 - Method and apparatus for adjusting laser's frequency

Info

Publication number: WO2019193587A1
Application number: PCT/IL2019/050368
Authority: WO
Inventors: Omri WOLF; Itamar GURMAN
Original assignee: Oryx Vision Ltd.
Priority date: 2018-04-05
Filing date: 2019-03-28
Publication date: 2019-10-10

Abstract

An optical arrangement is described for use in transmission of modulated signals, where the arrangement comprises: at least two light beam generating sources and an optical transmitter, and the optical arrangement is characterized in that: a) a first of the two-light beam generating sources is configured to be used in measuring under real time or near real time conditions an optical path extending within a cavity of a second of the at least two light beam generating sources; and b) results of the optical path measurements carried out by using the first light beam generating source, are applied for adjusting frequency modulation of signals generated by the second light beam generator, when these signals are transmitted by the optical transmitter.

Description

METHOD AND APPARATUS FOR ADJUSTING LASER ' S FREQUENCY

TECHNICAL FIELD

The present disclosure generally relates to the field of optical systems. More particularly, the present disclosure relates to systems implementing control of frequency modulation.

BACKGROUND

Optical coherent detection is a method of extracting information encoded as modulation of the phase and/o frequency of electromagnetic radiation in the wavelength band

v1 s rb1e or infrared liaht .

Coherent optical communication systems, typically utilize frequency modulation (hereinafter "FM") techniques in order to exchange communications throughout these systems. Optical radars, for example, utilizing linear FM (CHIRP) modulation to detect distance and speeds of targets, are based on the assumption that the frequency of the laser changes (i.e. tuning) with time in a predetermined manner. Any deviations of the modulated signal from the desired wave form might result in a decreased performance of the communication system. In particular, non-linearity in chirped optical radars might result in a reduced performance.

Lasers are devices widely used as transmitters in optical communication systems. The gain medium and the optical length of the laser cavity determine the exact operating frequency of the respective laser. Optical length is defined as the product of the physical length of the cavity and the refractive index of the medium within that cavity. Frequency tuning (or modulation) is typically (but not necessarily) achieved by changing the length of the laser cavity by, for example, mounting one of the end mirrors of the cavity on a piezoelectric actuator that is configured to move the mirror along an optical axis.

Accurate frequency control of optical signals is a difficult task to obtain as will be explained hereinbelow, yet applications requiring such an accurate frequency control, entail methods that are designed to obtain this goal. Prior art methods can be cumbersome, expensive, or difficult to implement.

As explained above, laser frequency is determined by several dependent variables, including: optical cavity physical length and gain medium characteristics. In the case of gas lasers, these variables may further include pressure, chemical composition, and temperature. For example, a system designed to generate linearly chirped optical signals utilizing a pulsed gas laser fitted with a piezoelectric actuator, will exhibit non-linearity behavior due to the fact that in a pulsed laser, the refractive index of the medium is changed during the pulse due to heating. This fact causes the optical length to change during the pulse, resulting in an undesirable frequency shift. In addition, any hysteresis in the piezoelectric actuator will further enhance the non-linearity.

There are quite a few publications in the art that are concerned with these issues, such as the following ones:

B.H. Sang and T.I Jeon describe in their article titled "Pressure-dependent refractive indices of gases by THz rime-domain spectroscopy" Optics Research Vol . 24, No. 25 pp . 29040-29047 (2016), a noncontact terahertz time-domain spectroscopy that was employed to measure pressure-dependent refractive indices of gases such as helium (He) , argon (Ar) , krypton (Kr) , oxygen (0₂) , nitrogen (N2) , methane (CH₄) , and carbon dioxide (CO2) . Although the studied gases differed in terms of their molecular structure, their refractive indices were strongly determined by their polarizability. D. A. Wojaczek, E. F. Plinski and J. S. Witkowski, in their article "Thermodynamic and optical parameters of the RF pulse excited slab-waveguide C02 laser", Optica Applicata, Vol . XXXV, No. 2, pp . 215 - 224 (2005), describe changes occurring to optical and thermodynamic parameters, such as a refractive index, pressure, and temperature on an RF pulsed CO2 slab-waveguide laser. Changes of the refractive index and pressure were measured with a Mach-Zehnder interferometer, and microphone, respectively. The article shows how an acoustic wave in the laser cavity modifies the pressure of the laser medium in the course of the pulse duration. The results of measurements allow explaining a line hopping effect observed during the laser plasma pulse evolution.

The present invention seeks to provide methods enabling to accurately measure the optical cavity length during lasing and affecting compensation thereof in order to obtain a desired modulation scheme.

SUMMARY

The disclosure may be summarized by referring to the appended claims .

It is an object of the present disclosure to provide a device and a method that allow measuring the optical length of a laser cavity by a first laser under real time conditions, and then using the resulting signal in order to enable an accurate adjustment of modulated signal (s) transmitted by a second laser associated with the cavity whose optical length was measured by the first laser.

Other objects of the present disclosure will become apparent from the following description.

According to a first embodiment of the present disclosure, there is provided an optical arrangement for use in transmitting modulated signals, the arrangement comprising:

at least two light beam generating sources; and an optical transmitter,

wherein the optical arrangement is characterized in that: a) a first of the two-light beam generating sources is configured to be used in measuring, under real time conditions, an optical path extending within a cavity of a second of the at least two light beam generating sources; and

b) results of the optical path measurements carried out by using the first light beam generating source, are applied for adjusting frequency modulation of signals generated by the second light beam generator, when the signals are transmitted by the optical transmitter .

The term "under real time conditions" as used herein throughout the specification and claims, should be understood to encompass both "under real time conditions" as well as "under near real time conditions".

According to another embodiment, the first light beam generating source is configured to emit light beams at a wavelength considerably lower than the wavelength at which the second light beam generating source is configured to emit light beams. Optionally, the first light beam generating source is configured to emit light beams at a wavelength which is less than lpm and the second light beam generating source is configured to emit light beams at a wavelength which is higher than 2pm.

By yet another embodiment, each of the first and the second light beam generating sources are based on different technologies. For example, the first light beam generating source can be a laser diode, whereas the second can be a gas laser. In another example, the first light generating source is one suitable for performing white light interferometry as means for measuring the optical length of the cavity of the second laser. In other words, the present invention encompasses an option where the first light beam generating source is not necessarily a laser.

According to still another embodiment, the optical arrangement further comprises a processor configured to receive data which relates to results of measurements of the optical path length and apply the received data for adjusting the frequency modulation of signals generated by the second light beam generator, when transmitted by the optical transmitter, in accordance with the latest measured optical length of the cavity of the second light beam generating source.

According to another aspect of the disclosure, there is provided a method for adjusting frequency modulations of signals transmitted by an optical transmitter of an optical arrangement that comprises at least two light beam generating sources and an optical transmitter, wherein the method comprises the steps of:

a) measuring, under real time conditions, by a first of the two-light beam generating sources, an optical path extending within a cavity of a second of the at least two light beam generating sources; and b) applying results of the optical path measurements for adjusting frequency modulation of signals generated by the second light beam generator, when the signals should be transmitted by the optical transmitter.

By yet another embodiment of this aspect of the disclosure, the first light beam generating source is configured to emit light beams at a wavelength considerably lower than the wavelength at which the second light beam generating source is configured to emit light beams. Optionally, the first light beam generating source is configured to emit light beams at a wavelength which is less than lpm and the second light beam generating source is configured to emit light beams at a wavelength which is higher than 2pm. In accordance with another embodiment of this aspect of the disclosure, the first light beam generating source may be a laser diode, and the second may be a gas laser.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate several embodiments of the disclosure and, together with the description, serve to explain the principles of the embodiments disclosed herein .

FIG. 1 illustrates a schematic view of an optical arrangement construed in accordance with the present disclosure;

FIG. 2A and 2C are schematic representations of changes incurred in interferometry signals emitted from the first laser in various cases, whereas FIG. 2B and FIG. 2D are schematic representations frequency changes in various cases;

FIG. 3A and FIG. 3B are schematic representations of two optional configurations construed in accordance with an embodiment of the present invention. FIG. 3A illustrates an embodiment of a heterodyne scheme whereas FIG. 3B presents an embodiment of a homodyne scheme; and

FIG. 4 exemplifies an embodiment of a method construed according to the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Some of the specific details and values in the following detailed description refer to certain examples of the disclosure. However, this description is provided only by way of example and is not intended to limit the scope of the invention in any way. As will be appreciated by those skilled in the art, the claimed method and device may be implemented by using other methods that are known in the art per se. In addition, the described embodiments comprise different steps, not all of which are required in all embodiments of the invention. The scope of the invention can be summarized by referring to the appended claims.

FIG. 1 presents a schematic view of an optical arrangement construed in accordance with an embodiment of the present invention. Assembly 50, presented in this example, comprises two light emitting sources (e.g. lasers), wherein the first laser 100 is a laser diode and the second laser is 200. Each of these two lasers is configured to emit light at a wavelength that is substantially different from the wavelength at which the other light emitting source is configured to emit light.

Let us now assume that in this example the first light emitting source is a laser diode configured to emit light a substantially shorter wavelength than the second wavelength at which the other light emitting source (a CO2 laser) is operative, say, the first laser operating wavelength is less than lpm, whereas the second laser operating wavelength is above 2pm. Element 210 is the output coupler of laser 200, which is designed in this example to be semi-transparent to light beams that are transmitted at both wavelengths at which laser 100 and laser 200 are operative. While the splitting ratio of element 210 for the wavelength of laser 200 is a critical design parameter of laser 200, there is no strict design requirement for the splitting ratio at the wavelength of laser 100. It is sufficient that enough light is able to pass, so as to allow light detection by photo detector 300. Element 220 is a rear mirror of the second laser which is configured to reflect the laser beams of both the first laser 100 and the second laser 200.

As mentioned above, element 300 is a photo detector, whereas element 400 is a beam splitter which is configured to reflect a portion of the light beam arriving from laser 100, while transmitting the other portion of that light beam. Element 500 is a dichronic mirror, being a mirror that has a significantly different reflection or transmission properties at two different wavelengths. In our example, mirror 500 is configured to reflect the laser beam emitted from the first laser 100 and to transmit the laser beam emitted from the second laser 200.

Light beam 600 is the frequency modulated light beam emitted from the second laser 200, that has been conveyed via mirror 500 and illuminates the system field of view. It is one of the goals of this disclosure to provide a means to accurately control the frequency modulation of light beam 600, thereby to obtain an optimal performance of the system.

700 designates the light beam emitted from the first laser 100 and conveyed to the output coupler 210 of the second laser 200, whereas 710 designates a light beam that is detected by photodetector 300 namely, the light emitted from the first laser and was reflected by output coupler 210 and reflected by mirrors 500 and 400.

Light beam 720 is a light beam that is also detected by photodetector 300. A portion of light beam 700 which has been forwarded by element 210, is reflected off rear mirror 220, passes via dichroic mirror 500 and beam splitter 400, before it reaches photodetector 300.

According to this embodiment, the first laser 100 is used to measure the present (actual) optical path length within the laser cavity of the second laser while the second laser 200 is operational (i.e. for lasing), by using any applicable method that is known in the art per se, such as interferometry. This signal is then used in a feedback loop to properly adjust the optical length of the second laser cavity in accordance with the value derived from the measurement carried by using the first laser, thereby enabling to achieve a more accurate frequency modulation for signals emitted by the second laser. The following Table 1 summarizes some of the differences between the laser used according to the embodiment of the present invention as a first laser, and the laser that is used according to this embodiment of the present invention as the second laser.

Table 1

The changes that occur in the actual optical cavity length may be divided into two main categories :

(1) Changes that occur due to deliberate optical cavity length manipulation, including but not limited to: physical movement of the mirrors, application/changes of electric and/or magnetic field, changes in temperature, acousto-opt eal manipulation, and the like. Such deliberate manipulations may be applied in order to achieve the desired frequency modulation scheme.

(2) Unplanned changes of the optical cavity length (deviations from sources), including but not limited to: thermal expansion/contraction, which the mechanical structure supporting the mirrors has undergone, refractive index changes due to heating of the medium (this effect should be understood to include also all other processes that are associated in one way or another with a temperature change, e.g. a change of pressure), nonlinear refractive index change due to strong oscillating electric fields (e.g. nonlinear kerr effect), parasitic non-linear effects occurring in the mechanical and/or electrical driving circuitry, etc. These unplanned changes might cause the actual frequency modulation scheme to differ from the desired scheme. Preferably, the desired scheme is a linear frequency modulation (CHIRP), whereas the unplanned effects might cause the modulation to become a non-linear one.

As was previously explained, one of the objects of the present invention is to provide a solution by which the optical length of the laser cavity is measured under real time conditions and the result (e.g. the resulting signal) of this measurement, is used to close a feedback loop, thereby enabling an accurate adjustment of the modulated signal transmitted by the second laser.

Let us assume that the interferometry signal, S, is represented by the following relationship:

S = sin (2nAnAL/Ao)

Where lo is the wavelength of the first laser and the product AnAL is the product of the differences in the index of detraction (n) and the physical length (L) , respectively. This product represents the change that the optical length of the second laser cavity has undergone.

Using the above relationship, one can achieve the two main objectives of the measurement carried out by using the first laser, namely :

1) the rate of change in the optical length correlates to the rate of change in the frequency modulation.

2) assuming a single mode operation, the total change in the optical length corresponds to the total change in 1/f (where f is the frequency) . In linear modulation applications, this parameter is known as the chirp depth or the chirp bandwidth.

Let us now turn to FIGs. 2A to 2D. In FIG. 2A, the signal designated by 800 is the interferometry signal emitted by the first laser. As may be seen in FIG. 2A, the interferometry signal exhibits an inconsistent frequency as a function of time, indicating that the optical path length of the second laser cavity is changing with time in a non-linear fashion.

FIG. 2B presents two curves, 850 and 860. Curve 850 illustrates the frequency change of the second laser, exhibiting the non-linearity presented in curve 800 of FIG. 2A, whereas curve 860 is a reference linear curve.

FIG. 2C demonstrates curve 900 is the interferometry signal emitted by the first laser, followed by a closed loop correction. In this case, the signal designated by 900 exhibits a constant frequency .

In FIG. 2D, curve 950 illustrates the frequency change, which, in this case, exhibits a near perfect linear chirp. In the present discussion, it is assumed that the changes in the optical length of the cavity that occur during measurements which are carried out at the first wavelength (i.e. the wavelength at which the first laser emits light, which is different from the wavelength at which the second laser emits (i.e. the second wavelength), these changes may be correlated at the second wavelength. In other words, it is assumed that the interferometry pattern of the light emitted from the first laser includes sufficient information to allow affecting corrections to the deviations that occur in second laser transmissions from the desired modulation.

FIG. 3A and FIG. 3B are two schematic representations of two optional topologies construed in accordance with an embodiment of the present invention. These FIGs. assume that the main laser (i.e. the second laser) is a CO2 gas laser. However, as any person skilled in the art would appreciate, this assumption was made only for illustrative purposes, and any other applicable laser may be used instead .

FIG. 3A presents a preferred embodiment of a heterodyne scheme is illustrated, where the beam reflected from the 'output coupler' along path a and the beam reflected from the 'end mirror' along the path b, interfere with each other. In this case, the piezoelectric transducer (used for deliberate manipulation of the optical length) can be on either at the end mirror or at the output coupler. The difference in the optical paths is equal to twice the optical length of the cavity. This method is very robust to changing environmental conditions since the optical path difference is only at the laser cavity. This method preferably requires a measurement laser (i.e. the first laser) having a coherence length of at least twice the cavity length.

FIG. 3B presents another embodiment implementing homodyne scheme, where the beam that transverses the cavity interferes with a third beam. In other words, path b interferes with path g. The piezoelectric transducer in this case should be at the end mirror. This scheme may be used for example in case the reflection from the output coupler is not available or if one wants to use a measurement laser with an arbitrary coherence length, since path Y (as illustrated in the figure) can be adjusted to be nearly identical to path b.

A further advantage of the solution provided by the present invention is that it allows to provide direct measurement of the chirp rate (also known as chirp bandwidth) . The chirp rate is a system parameter used to extract target range and velocity. It may therefore be critical to have an accurate knowledge of the chirp rate. The proposed solution enables direct measurement of the chirp rate .

FIG. 4 exemplifies an embodiment of a method for carrying out the present invention, whereby the frequency modulation of signals transmitted by an optical transmitter of an optical arrangement, is adjusted. The embodiment exemplified herein comprises the steps of: providing an optical arrangement that comprises at least two light beam generating sources and an optical transmitter (step 1000) . The next step comprises measuring, under real time or near real time conditions, by a first of the two-light beam generating sources, an optical path extending within a cavity of a second of the at least two light beam generating sources (step 1100) .

Then, assuming that the interferometry signal S detected by the photodetector is presented by the following relationship:

S = sin (2nAnAL/Ao) where lo is the wavelength of the first laser and the product AnAL is the product of the differences in the index of detraction (n) and the physical length (L) , respectively. This product represents the change in the optical length which the second laser cavity has undergone .

Using the above relationship, calculating the rate of change in the optical length that corresponds to the rate of change in the frequency modulation (step 1200), and calculating the total change in the optical length that corresponds to the total change in frequency (step 1300) .

Next, applying the results of the optical path measurements for adjusting the frequency modulation of signals generated by the second light beam generator, for their transmission by the optical transmitter (step 1400) .

According to another embodiment, the optical arrangement is comprised within a single device, e.g. a device incorporating the first laser and its associated optics that are included within the enclosure of the second laser. In other words, according to this embodiment, a single laser device incorporates all (or at least some of) the elements of the first laser interferometry sub system within the envelope of a second laser. For example, the first laser is as a diode laser and this diode laser and its associated optics are mounted within a gas reservoir of a CO2 laser, being the second laser . Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. For example, the entire optical path, starting at the laser diode and ending at the photo detector, excluding the second laser cavity being measured, may be implemented by using fiber optics. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

Claims

WHAT IS CLAIMED IS:

1. An optical arrangement for use in transmitting modulated signals, said arrangement comprising:

at least two light beam generating sources; and

an optical transmitter,

wherein the optical arrangement is characterized in that: a) a first of the two-light beam generating sources is configured to be used in measuring, under real time conditions, an optical path extending within a cavity associated with a second of the at least two light beam generating sources; and

b) results of the optical path measurements carried out by using the first light beam generating source, are applied for adjusting frequency modulation of signals generated by the second light beam generator, when transmitted by the optical transmitter.

2. The optical arrangement of claim 1, wherein the first light beam generating source is configured to emit light beams at a wavelength lower than the wavelength at which the second light beam generating source is configured to emit light beams.

3. The optical arrangement of claim 2, wherein the first light beam generating source is configured to emit light beams at a wavelength which is less than lpm and the second light beam generating source is configured to emit light beams at a wavelength which is above 2pm.

4. The optical arrangement of claim 1, wherein the first light beam generating source is a laser diode and the second light emitting sources is a gas laser.

5. The optical arrangement of claim 1, further comprising a processor configured to receive data which relates to results of the measurements of the optical path length, and apply the received data for adjusting the frequency modulation of signals generated by the second light beam generating source when transmitted by the optical transmitter in accordance with the latest measured optical length of the cavity associated with the second light beam generating source.

6. A method for adjusting frequency modulation of signals transmitted by an optical transmitter of an optical arrangement that comprises at least two light beam generating sources and an optical transmitter, said method comprising the steps of:

a) measuring under real time conditions, by a first of the two-light beam generating sources, an optical path extending within a cavity of a second of the at least two light beam generating sources; and b) applying the results thus obtained of the optical path measurements for adjusting frequency modulation of signals generated by the second light beam generator, when said signals are transmitted by the optical transmitter.

7. The method of claim 6, wherein the first light beam generating source is configured to emit light beams at a wavelength lower than the wavelength at which the second light beam generating source is configured to emit light beams.

8. The method of claim 7, wherein the first light beam generating source is configured to emit light beams at a wavelength which is less than lgm and the second light beam generating source is configured to emit light beams at a wavelength which is above 2gm.

9. The method of claim 6, wherein the first light emitting sources is a laser diode and the second light emitting sources is a gas laser.