WO2018149508A1

WO2018149508A1 - Automatic and programmable signal generation method in the radiofrequency, milimeter wave and terahertz frequency regions with low phase noise, wide frequency tuning range, high modulation bandwidth and high frequency resolution

Info

Publication number: WO2018149508A1
Application number: PCT/EP2017/053773
Authority: WO
Inventors: Rubén CRIADO SERRANO; Jesús PALACÍ LÓPEZ; Cristina DE DIOS FERNÁNDEZ; Pablo ACEDO
Original assignee: Luz Wavelabs SL
Current assignee: Luz Wavelabs SL
Priority date: 2017-02-20
Filing date: 2017-02-20
Publication date: 2018-08-23
Anticipated expiration: 2019-08-20

Abstract

A method for automatically generating signals in the radiofrequency, milimeter wave and terahertz frequency regions with low phase noise, wide frequency tuning range, high modulation bandwidth and high frequency resolution is hereby provided. The method of the invention is related to Optical Injection Locking (OIL) techniques and allows proper injection of lasers, a slave laser and a master laser, by determining the most suitable injection point which is defined from measurements and calculations based on the monitoring of voltage values in the electrodes of the slave laser and sweeping of parameters related to the operational conditions of the slave laser. The method of the invention hereby disclosed provides automatic and programmable signal generation from radiofrequency up to Terahertz frequencies, simultaneously providing low phase noise, high modulation bandwidth, wide frequency range and high frequency resolution.

Description

AUTOMATIC AND PROGRAMMABLE SIGNAL GENERATION METHOD IN THE RADIOFREQUENCY. MILIMETER WAVE AND TERAHERTZ FREQUENCY REGIONS WITH LOW PHASE NOISE. WIDE FREQUENCY TUNING RANGE. HIGH MODULATION BANDWIDTH AND HIGH FREQUENCY RESOLUTION

D E S C R I P T I O N

OBJECT OF THE INVENTION

The invention hereby disclosed belongs to the field of electromagnetic signal generators; more precisely in the frequency regions covering radiofrequency, microwave, millimeter wave and Terahertz ranges.

The object of the invention allows, amongst others, a method for automatic and programmable signal generation from radiofrequency up to Terahertz frequencies, simultaneously providing low phase noise, high modulation bandwidth, wide frequency range and high frequency resolution.

BACKGROUND

Optical injection has many applications, being remarkable those aimed to signal processing, wavelength stabilization and metrology.

US6359913B1 discloses an injection locking system for lasers; in particular, a signal from a master laser is phase modulated and injected into a slave laser. The phase difference φ is maintained at zero by means of a phase locked loop. By maintaining the phase difference φ at zero, the frequency drift is compensated by maintaining the frequency within a predetermined locking range. Currently there are forms of automating optical injection, for example using the slave laser optical output power, it is given that it decreases when the injection takes place. In this case a photodiode may be used to monitor the power output, thus requiring additional equipment. The resolution provided is not accurate, hence when little power is injected it is not possible to differentiate neither between any injection and non-injection situations. The voltage between electrodes of the slave laser is reduced when the injection is produced and there are two main forms of measuring it. Although,, in principle, in this case no additional equipment might be required, in order to get a great resolution either the master or the slave laser should be alternatively turned on and off in order to make the detection balanced; or its current should be modulated varying the phase of the radiation emitted by the slave laser. These modulations limit the applicability of the technique, since it does not allow the slave laser to operate in a stable form. In addition, additional equipment may be required to produce these modulations. This prevents these techniques to be feasible in applications such as wireless communications, for example, or any other application that requires from the laser to be switched on all the time or having its phase stable.

This type of signal generation structures are used in different laboratories at R & D level, some of them use a type of optical comb, whereas others perform a fine tuning with an electro-optical modulator, some maintain the optical injection locking using a PLL and others not, etc ...) to this day there is no knowledge of being able to carry out this without using PLLs (which are expensive and complex). Traditional methods of signal generation in the radiofrequency, microwave, and millimeter wave and terahertz region rely on electronic approach or photonic approach. While the electronic approach (frequency multiplication) offers low frequency noise and high frequency resolution, it has limited bandwidth modulation and frequency tuning range. Photonic methods for signal generation in this frequency ranges are based on optical down-coversion (two optical modes spaced by the desired frequency are down-converted by means of a photomixing or photodetection). These methods are complementary to electronic approaches, in the sense that they provide high modulation bandwidth, wide frequency tuning range but poor performance in terms of frequency resolution and phase noise. Being the phase noise characteristics a requirement for some applications like communications or instrumentation, photonic approaches are not commonly used for these applications.

Another type of methods for signal generation up to Terahertz range is based in the combination of both electronic and photonic techniques. In these cases, the two optical modes are extracted from an optical frequency comb, what gives a very high coherence between optical modes, thus a higher phase noise performance. There are two major stages in these systems: an optical frequency comb and an optical modes selection stage. The optical comb can be generated in a variety of ways and can provide different features in terms of optical span, frequency spacing and frequency spacing tunability, optical flatness whereas the optical modes selection stages can be based on optical filtering or optical injection locking. These kinds of systems are able to simultaneously provide all the previous features: wide frequency range, high modulation bandwidth, low phase noise and high frequency resolution.

Several developments related to this problem have been disclosed; in this sense the applicant is aware of the disclosure of WO0233818A2 where a frequency synthesizer for generating wideband signals for zero Hz up to frequencies in the millimeter wave band is disclosed, said frequency synthesizer provides the ability for arbitrarily tine increments in output frequency. The synthesizer has an optical comb generator that provides a set of reference signals at predetermined intervals which are fed to two or more laser devices, at least one of which is a frequency selection and translation device capable of selecting a particular output line from the comb generator and shifting it by some desired frequency. The outputs front the laser devices are then combined and detected using a photodetector. WO2017010603 discloses a device and a method based on PDH (Pound Drever Hall technique) for performing stabilization of the overall frequency of an optical comb of a femtosecond laser by using an optical mode directly extracted from the optical comb and, more specifically. This document is mainly aimed to a device and a method for performing stabilization of the overall frequency of an optical comb of a femtosecond laser by using an optical mode directly extracted from the optical comb, in which the overall frequency of an optical comb of a femtosecond laser is stabilized by using an optical mode directly extracted from the optical comb and monochromatic laser and pulse having excellent frequency stability and linewidth are generated from the stabilized optical comb. The device disclosed comprises a femtosecond laser light source comprising in turn a length adjustment element and any one selected from a pump laser and an acousto-optic modulator and is for outputting a femtosecond laser optical comb, an optical mode extraction unit for extracting, from the femtosecond laser optical comb, a first optical mode and a second optical mode having frequencies that are different from each other, an injection-locking unit for generating first monochromatic light by injection-locking the first optical mode to a first cascade laser and for generating second monochromatic light by injection-locking the second optical mode to a second cascade laser and a control unit comprising for controlling the length adjustment element by means of generating a first optical comb control signal on the basis of a resonant frequency of the high sharpness- cavity and the first monochromatic light and for controlling the pump laser or the acousto- optic modulator, provided in the femtosecond laser light source, by means of generating a second optical comb control signal on the basis of the resonant frequency of the high sharpness-cavity and the second monochromatic light. The method disclosed in WO2017010603 requires form dithering modulation techniques which may produce spurious data.

US5379309 is based on heterodyne techniques and discloses a signal generator including a mode-locked laser having multiple optical modes characterized by respective mode frequencies, plural tuned lasers, each of the plural tuned lasers being tuned to a respective optical frequency corresponding to respective ones of the mode frequencies, wherein a difference in the optical frequencies of different ones of the plural tuned lasers corresponds to a desired output frequency, apparatus for injection-locking the plural tuned lasers to corresponding ones of the plural optical modes of the mode-locked laser, and apparatus for combining the optical outputs of the plural tuned lasers whereby to generate at least an output signal having an output frequency equal to the difference in the optical frequencies of different ones of the plural tuned lasers. The plural optical modes of the mode-locked laser span a frequency band at least hundreds of GHz wide, and the output frequency is in a band encompassing millimeter and submillimeter wavelengths. The plural tuned lasers include a pair of C.W. lasers each narrowly tuned to a respective optical frequency corresponding to a frequency difference therebetween equal to the output frequency. Injection-locking is accomplished by coupling optical radiation from an optical cavity of the mode-locked laser to an optical cavity of each one of the plural tuned lasers. An up-converter modulates the optical output of one of the plural tuned lasers with the baseband signal prior its being combined by the apparatus for combining with an electrooptical phase modulator having an optical input connected to the one tuned laser and an optical output connected to the combining apparatus and an electrical control input connected to receive the baseband signal. Nevertheless, all existing system in this category needs of bulky and expensive equipment, highly qualified staff to operate the system and are not automatic or programmable, what limit their possibilities to actual applications. Nowadays there is no development allowing to obtain the above mentioned results without implementing PLLs or using dithering techniques; both being based on power measured whilst the first requires from expensive equipment, highly qualified staff to operate, and the latter rendering non-real results.

DESCRIPTION OF THE INVENTION The object of the invention is aimed to a method for signal generation in the RF, mm-wave and terahertz domains, based on optoelectronic techniques, able to simultaneously achieve low-phase noise, wide frequency range, high modulation bandwidth and high frequency resolution. On top on that, the novelty of our invention allows a compact system, and especially totally automatic and programmable operation.

In the method of the invention, optical injection is performed using two lasers: a master laser and a slave laser. The technique provided mainly consists of introducing (or injecting) the optical signal (or emitted light) of the master laser into the slave laser. When the emission frequencies of both lasers are close to each other, under injection conditions, the slave laser will emit at exactly the same frequency as the master laser emits (instead of the one it would emit on its own if there was no injection), also inheriting its physical properties, such as noise: this process is known as optical injection locking (OIL), a process by which a master laser is injected into a slave. Hence, Optical injection locking (OIL) is mainly a process by which a master laser is injected into a slave: If the slave is tuned within the locking range (which depends on the power injected and is between hundreds of MHz and a few GHz), a frequency identical to the master of the slave emerge, filtering the rest of the slave spectrum. In addition, if it stays at a fixed point within that blocking range, the phase does not change either. Unlike in techniques known in the art, where it can be found that the laser is kept fixed within the range by PLLs, the method of the invention uses voltage measurements between anode and cathode so it is possible to extract a bimodal signal that allows knowing the locking range and a ramp with the variation of the phase. This signal, of a magnitude much smaller than the offset signal, is differentiated with respect to previous samples in order to monitor its variability. This method is not affected by the offset value, which is especially relevant when the laser is tuned in current and not in temperature. It is also immune to fluctuations in the offset value due to temperature changes or long term degradation of the laser. That signal is the basis for putting the lasers in place at all times, and also calibrate that information as the system works to keep it updated at all times.

The method of the invention may be implemented in a system like a high bandwidth signal generator especially adapted and ranging from MHz to THz and based on RF-photonic hybrid technology. Two optical modes are extracted from an optical comb to be detected in a photo-mixer or photodiode and generate the desired frequency which will be equal to the spacing between the modes, these optical modes are filtered and amplified in one step using optical injection locking techniques.

The optical comb may be tunable in repetition frequency, so the selection of different optical lines (coarse tuning) together with the change of the repetition frequency (fine tuning) results in the possibility of continuously tuning the entire band of Interest with high frequency resolution.

DESCRIPTION OF THE DRAWINGS

In order to complement the description being made and in order to aid a better understanding of the features of the invention, according to a preferred example of practical embodiment thereof, a set of drawings is attached as an integral part of said description in which, with an illustrative and non-limiting character, the following has been represented:

Figure 1. Depicts a flowchart of the method of the invention.

Figures 2a -2d. Show respective diagrams depicting the injection process and the voltage difference and determination of the threshold.

PREFERRED EMBODIMENT In a preferred embodiment of the method of the invention depicted in figure 1 ; a slave laser, which is used to carry out the OIL process (for example a set of temperature and current), is turned on; then a reference or master laser, which is another laser, is also turned on injecting its light into the slave laser as seen in figure 2a. Voltages in the electrodes of the slave laser are measured and said value/s is/are kept. By sweeping some parameters of the operating conditions of the slave laser, usually current and/or temperature of the slave laser, changes in the emission frequency are generated, which will get either closer or further from the master laser's emission frequency as seen in figure 2b.

The voltage in the electrodes is measured every single time that at least one of any one of the parameters of the operating conditions of the slave laser is modified -as seen in figure 2c- said voltage is expected to drop once the OIL process occurs (see figure 2c), namely when the frequency of the of the slave laser is close to that of the master laser and it locks into it copying its exact properties. However, this drop is so very small that only variances detected in the voltage, but not its average value, provide the resolution required to detect the injection point; to do so, previous voltage value/s is/are respectively subtracted from actual one voltage value/s until the sweep is over. The calculation is based on the variance of the voltage and comprises differentiating consecutive voltage values when sweeping parameters related to operating conditions of the slave laser.

At this point it is required to look for any of the slave laser operating conditions that originated a larger voltage variation, namely larger differences: since this is considered to be an injection point, the point where the slave laser is injected. Once the injection point is determined, the slave laser moves into the injection point.

Deviations in the electrodes voltage are tracked to keep it injected, using a threshold which may be defined from previous variance measurements. The threshold value may be defined by turning on and off the master laser and calculating a differential voltage change around an optimum injection point, generating a bi-pulse response whose optimum injection point falls right in the middle.

Claims

1. Automatic and programmable signal generation method in the radiofrequency, millimeter wave and terahertz frequency regions with low phase noise, wide frequency tuning range, high modulation bandwidth and high frequency resolution, the method comprising:

• turning on a slaver laser setting at least temperature and/or current values for the slave laser,

• turning on a master laser, and

• start injecting the light emission of the master laser into the slave laser, the method being characterized by comprising:

• measuring voltage in the electrodes of the slave laser,

• sweeping at least one of temperature and current parameters of the slave laser while measuring voltage values in the electrodes of the slave laser when at least of said parameters changes, and

• determining an injection point when voltage values in the electrodes of the slave laser drop, said determination comprising:

^■ a calculation based on the variance in the voltage, and

^■ a determination of parameters values related to slave laser operating conditions, said parameters being determined when a larger voltage variation occurs.

2. Method according to claim 1 wherein the calculation based on the variance of the voltage comprises differentiating consecutive voltage values when sweeping parameters related to operating conditions of the slave laser.

3. Method according to claim 1 further comprising tracking the voltage of the electrodes of the slave laser to keep it injected.

4. Method according to claim 3 wherein the tracking is performed using predefined threshold value.

5. Method according to claim 4 wherein the threshold value is defined by turning on and off the master laser and calculating a differential voltage change around an optimum injection point, generating a bi-pulse response whose optimum injection point falls in the middle.

6. Method according to any one of claims 1 to 5 further comprising readjusting the slave laser parameters if the voltage goes out of a certain range to compensate for the deviation and keep the slave laser injected.

7. Method according to any one of claims 1 to 6 wherein the parameters related to slave laser operating conditions are selected from: temperature and current.