WO2015189779A2

WO2015189779A2 - Mode locked laser (mll) for generating a wavelength stabilized depletion pulse and method thereof

Info

Publication number: WO2015189779A2
Application number: PCT/IB2015/054378
Authority: WO
Inventors: Prabhakar ANIL; Mayor SATYAJIT; Krishnamoorthy SHREE
Original assignee: National Centre For Biological Sciences; Indian Institute Of Technology, Madras
Priority date: 2014-06-10
Filing date: 2015-06-10
Publication date: 2015-12-17
Also published as: WO2015189779A3

Abstract

The present disclosure relates to a mode locked laser fiber to generate wavelength stabilized depletion pulse laser and a system for synchronizing wavelength stabilized depletion pulse laser with a master laser. In one embodiment, a predetermined fraction of the master laser pulses are fed as input to a mode locked laser fiber that generates a wavelength stabilized slave pulse laser. The wavelength stabilized salve pulse laser is then amplified and fed to a second harmonic generation (SHG) crystal. The SHG crystal converts the amplified slave pulse laser into the depletion pulse laser synchronized with the excitation laser.

Description

"MODE LOCKED LASER (MLL) FOR GENERATING A WAVELENGTH STABILIZED DEPLETION PULSE AND METHOD THEREOF"

TECHNICAL FIELD

The present disclosure generally relates to fiber lasers, and more particularly relates to mode locked lasers (MLL) for synchronizing two or more pulsed lasers to generate a train of wavelength stabilized optical slave laser pulses from master laser pulses.

BACKGROUND

Generally, imaging with visible light is a preferred means of visualizing microscopic samples like biological samples. In traditional microscopy, resolution is limited by the diffraction limit of light. Stimulated emission depletion (STED) microscopy uses fluorescent properties of molecules to image the biological samples beyond the optical diffraction limit. In STED microscopy, fluorescent molecules in the samples undergo transition from a higher energy state to a lower energy state by stimulated emission, using a light of particular wavelength incident on certain regions to achieve resolution better than the traditional confocal microscopy. Better resolution is achieved with higher laser power levels. Unfortunately, the molecules are subjected to a photo-bleaching process due to higher laser powers, which may also result in burning and causing damage to the samples.

Gas or solid state lasers, with the active mode locking (MLL) technique, provide pulses with high power and narrow pulse widths. However, these lasers are expensive, have heating issues and involve high cost of maintenance. Furthermore, these lasers fail to produce synchronized pulses with minimum inter-pulse jitter for given pulse repetition rates. MLL constructed with optical fibres or mode-locked fibre lasers, provide an attractive alternative to gas or solid state lasers. Fiber lasers are compact, convenient and easy to maintenance with little heating issues. Therefore, fiber based MLL can be used to synchronize two or more pulsed lasers and generate high power optical pulses with sub-nanosecond pulse width at a predefined wavelength. Consequently, those skilled in the art will appreciate the present disclosure that provides many advantages and overcomes all the above and other limitations.

SUMMARY

The shortcomings of the prior art are overcome and additional advantages are provided through the present disclosure. Additional features and advantages are realized through the techniques of the present disclosure. Other embodiments and aspects of the disclosure are described in detail herein and are considered a part of the claimed disclosure.

Accordingly, the present disclosure relates to a mode locked laser (MLL) for generating a wavelength stabilized depletion pulse train synchronized to optical pulses from a master excitation laser. The MLL comprises a pump laser source for generating plurality of optical energy pulse. Further, the MLL comprises an optical fiber ring cavity coupled with the pump laser. The optical fiber ring cavity comprises an electro-optic modulator (EOM) configured to receive a predetermined fraction of optical signals from a master laser source and convert the received fraction of optical signals into corresponding optical pulses of a depletion laser. Further, the optical fiber ring cavity comprises an optical gain medium coupled with the EOM. The optical gain medium is configured to receive energy from a broadband pump laser, multiplexed with slave optical pulses, and amplify the slave optical pulses. Furthermore, the optical fiber ring cavity comprises a wavelength selective filter coupled to the optical gain medium via a circulator, the wavelength selective filter is configured to filter the amplified slave optical pulses and the pump laser, thereby generating a wavelength stabilized slave depletion pulse. The generated wavelength stabilized slave depletion pulse is of predetermined wavelength with a repetition rate matching with repetition rate of the received master excitation laser pulses.

Further, the present disclosure relates to a method of generating a wavelength stabilized optical signal by operating a mode-locked fiber laser. The method comprises receiving a predetermined fraction of master laser pulses and converting into slave laser pulses of predetermined wavelength having repetition rate matching with repetition rate of the received master laser pulses. Further, the method includes multiplexing of the slave optical pulses with energy from a broadband pump laser, and co-propagating them in a gain fibre, thereby amplifying the multiplexed depletion pulses to generate a wavelength stabilized optical pulse train having a predetermined wavelength as determined by an optical filter.

Furthermore, the present disclosure relates to a fiber laser system for generating synchronized optical pulses. The system comprises a mode-locked laser (MLL) configured to receive a fraction of master laser pulses having a predetermined wavelength to generate a wavelength stabilized slave laser pulses. Further, the system comprises an optical amplifier communicatively coupled with the MLL. The optical amplifier is configured to receive and amplify the wavelength stabilized slave laser pulses to generate amplified wavelength stabilized slave pulses. Furthermore, the system comprises a second harmonic generation (SHG) crystal communicatively coupled with the optical amplifier. The SHG crystal is configured to convert the amplified slave pulses into a predetermined secondary wavelength to act as depletion laser pulses synchronized with the master excitation laser pulses.

The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present disclosure are set forth with particularity in the appended claims. The disclosure itself, together with further features and attended advantages, will become apparent from consideration of the following detailed description, taken in conjunction with the accompanying drawings. One or more embodiments of the present disclosure are now described, by way of example only, with reference to the accompanied drawings wherein like reference numerals represent like elements and in which: Figure 1A illustrates a fiber laser system in accordance with an embodiment of the present disclosure;

Figure IB illustrates an internal block diagram representation of an excitation laser source configured in the fiber laser system, in accordance with an embodiment of the present disclosure;

Figure 2 illustrates an optical circuit for a wavelength stabilized active mode locked fiber laser in accordance with an embodiment of the present disclosure;

Figure 3 illustrates an optical amplifier in accordance with an embodiment of the present disclosure; and

Figure 4 illustrates a graphical representation of spectrum of the excitation and depletion lasers in accordance with an embodiment of the present disclosure.

The figures depict embodiments of the disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the disclosure described herein.

DETAILED DESCRIPTION

While the disclosure is susceptible to various modifications and alternative forms, specific embodiment thereof has been shown by way of example in the drawings and will be described in detail below. It should be understood, however that it is not intended to limit the disclosure to the particular forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternative falling within the spirit and the scope of the disclosure.

The terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a setup, device or method that comprises a list of components or steps does not include only those components or steps but may include other components or steps not expressly listed or inherent to such setup or device or method. In other words, one or more elements in a system or apparatus proceeded by "comprises... a" does not, without more constraints, preclude the existence of other elements or additional elements in the system or apparatus.

Accordingly, the present disclosure relates to a mode locked laser (MIX) for generating wavelength stabilized slave depletion pulses synchronized to master excitation laser pulses. The MLL comprises a pump laser source for amplifying optical energy. The pump laser source is connected to an optical fiber ring cavity. The optical fiber ring cavity receives the optical energy from the pump laser source. The optical fiber ring cavity comprises an electro-optic modulator (EOM) configured to receive electrical pulses derived from a master laser source and convert the electrical pulses into corresponding depletion optical pulses. Further, the optical fiber ring cavity comprises an optical gain medium coupled to the EOM. The optical gain medium is configured to receive and amplify multiplexed optical pulses using energy from the pump laser source. Furthermore, the optical fiber ring cavity comprises a wavelength selective filter coupled to the optical gain medium via a circulator. The wavelength selective filter is configured to filter the amplified multiplexed optical pulses to a predetermined wavelength. The filtered pulses are coupled back to the optical fiber ring cavity to generate wavelength stabilized optical pulses. The generated wavelength stabilized optical pulses, referred to as slave laser pulses, are of predetermined wavelength with a repetition rate matching with repetition rate of the received master laser pulses.

Further, the present disclosure relates to a method of generating a wavelength stabilized optical signal by operating a mode-locked laser. The method comprising generating, by a master laser source, a plurality of optical energy pulses. Also, the method comprises receiving, by a optoelectronic radio frequency (RF) circuit, a fraction of master laser pulses and feeding it as electrical pulses to an electro optic modulator (EOM). Furthermore, the method comprises of the EOM receiving said electrical pulses and converting it into slave optical pulses having a repetition rate matching with repetition rate of the received master laser pulses. Further, the method includes multiplexing, by an optical gain medium, of the slave optical pulses and the pump laser source, and amplifying the multiplexed the slave optical pulses with energy from the pump laser source. Furthermore, the method includes generating, by a wavelength selective filter, wavelength stabilized optical pulses having a predetermined wavelength by filtering the amplified multiplexed slave optical pulses.

Furthermore, the present disclosure relates to a fiber laser system for generating synchronized high power optical pulses. The system comprises a mode-locked laser (MLL) configured to receive a predetermined fraction of master laser pulses having a first predetermined wavelength and generate wavelength stabilized slave optical pulses. Further, the system comprises an optical amplifier communicatively coupled with the MLL. The optical amplifier is configured to receive and amplify the wavelength stabilized slave optical pulses to generate amplified wavelength stabilized slave optical pulses. Furthermore, the system comprises a second harmonic generation (SHG) crystal communicatively coupled with the optical amplifier. The SHG crystal is configured to convert the amplified wavelength stabilized slave optical pulses into a second predetermined wavelength, referred to as the depletion laser pulses that are synchronized with the master excitation laser pulses. The second predetermined wavelength of the depletion laser pulses exceeds the first predetermined wavelength of the master laser pulses by a predetermined value.

In the following detailed description of the embodiments of the disclosure, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration specific embodiments in which the disclosure may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the disclosure, and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the present disclosure. The following description is, therefore, not to be taken in a limiting sense.

Optical imaging systems such as simulated emission depletion microscopy require the ability to synchronize optical pulses of an excitation laser with the optical pulses of a pulsed depletion laser. The depletion laser is typically required to have high power optical pulses, with a short pulse width and a fixed wavelength and minimum timing jitter. The present disclosure relates to a mode locked fibre laser configured to generate a pulsed depletion laser, or slave laser, in synchronization with the excitation laser, or the master laser. Further, the present disclosure relates to a method and a system for generating the pulsed depletion laser synchronized with the excitation laser.

Figure 1A illustrates a fiber laser system in accordance with an embodiment of the present disclosure.

As shown in figure 1A, the laser system (100) comprises of at least a laser synchronizing unit (101) and an excitation or master laser source (102). The master laser source (102) is configured to generate a plurality of excitation or master laser pulses of first predetermined wavelength and transmit fraction of the generated master laser pulses to the laser synchronizing unit. In one embodiment, the master laser source may be an Argon ion laser, a titanium (Ti) sapphire solid state laser or any other pulsed laser sources known in the art.

The master laser source (102) is communicatively coupled to a photo detector (104) that receives and processes a fraction of the plurality of master laser pulses transmitted by the master laser source (102). The photo detector (104) processes the master laser by converting the plurality of optical signals associated with the master laser into a plurality of electrical signals. The laser synchronizing unit (101) receives the plurality of electrical signals and converts the received signals into pulsed depletion laser pulses which are synchronous with the master laser pulses.

The laser synchronizing unit (101) comprising at least a radio frequency (RF) driving circuit (106), a mode-locked laser (MLL) (108), an optical amplifier (110) and a second harmonic generation crystal (SHG) (112). The RF driving circuit (106) receives the plurality of electrical signals from the detector (104) and converts the received electrical signals into plurality of optical signals. The electrical signals generated by the RF circuit (106) are fed as input to the MLL (108).

The MLL (108) is an active mode locked laser capable of emitting short pulses in the order of picoseconds to less than few femtoseconds. The short pulses will repeat at a predefined repetition rate during a predetermined round trip time. Round trip time is the time taken by the optical pulse to complete one round trip within an optical ring cavity comprised within the MLL.

The MLL (108) acts as a slave laser, and is configured to generate wavelength stabilized optical pulses having a predetermined wavelength, short pulse width and repetition rate matching with the repetition rate of the master laser pulses. The wavelength stabilized depletion pulse is then amplified by the optical amplifier (110).

The optical amplifier (110) is configured to receive and amplify the wavelength stabilized optical pulses from the slave laser (108) and generate amplified wavelength stabilized optical slave pulses or amplified optical pulses. In one embodiment, the optical amplifier is a master oscillator power amplifier (MOP A). The optical amplifier MOPA (110) may be built using one or more stages of double clad fiber (DCF) and large mode area (LMA) fiber. The slave optical pulses are generated with a predetermined wavelength such that the wavelength of the slave optical pulses exceeds the wavelength of the master laser pulse by a predetermined value. The amplified wavelength stabilized depletion pulse is then fed to the SHG crystal (112) for further processing.

The SHG crystal (112) converts the amplified wavelength stabilized optical pulses into a second predetermined wavelength of the slave optical pulses, synchronized with the master laser pulse. In one embodiment, the SHG crystal (112) is a Periodically Poled Lithium Niobate (PPLN) crystal. In another embodiment, the SHG crystal (112) may be any other crystal known in the art. Fiber coupled SHG crystal achieves efficiencies as high as 20%, however, the efficiency may vary depending on the intensity of the optical pulse.

Figure IB illustrates an internal block diagram representation of a master laser source or an excitation laser source (102) configured in the fiber laser system (100), in accordance with an embodiment of the present disclosure. As shown in the figure IB, the excitation laser (102) comprises a gain medium (114) and a dispersive element (116). The dispersive element (116) is configured to be electronically tuned to generate a better synchronize pulses. In one embodiment, speed of light for the excitation laser cavity is measured by the refractive index η of the dispersive element (116), which depends on the wavelength of light λ. The optical pulse in the excitation laser (102) circulates between a gain medium (114) and a dispersive element (116), and the path of the optical pulse is a dotted line (120), as shown in the Figure IB. In one embodiment, changing the wavelength of light λ, changes the speed of light in the laser cavity which in turn changes the repetition rate of output excitation (master) optical pulses sent to the depletion (slave) laser.

In one embodiment, a controller (118) is configured to alter the dispersive element (116) in and out of the laser cavity, and thus control the effective refractive index of the excitation laser pulses. The change in the effective refractive index, changes the repetition rate of output pulses from the excitation laser (102). Thus, the controller (118), by controlling the wavelength of light in the cavity tunes the repetition rate of the output pulses of the excitation laser (102). The output pulses from the excitation laser (102) drive the depletion laser. In one embodiment, when the repetition rate of the excitation laser pulses matches with one of the cavity round trip time of the depletion laser and a multiple of the cavity round trip time, then the depletion laser pulses become sharper. Thus, the width of the depletion laser pulses becomes a less than hundred picoseconds, which is approximately five times smaller than the pulse width of the mode-locked fiber depletion laser without this fine-tuning. The fine control of the timing of the excitation pulses to match the cavity round trip time of the depletion laser is achieved by controlling the effective refractive index seen by the optical pulse in the excitation laser cavity.

In another embodiment, an acousto-optic modulator or AOM (119) receives the input from the master laser (102) and changes its wavelength by a predetermined amount. The output of the AOM (119) then acts as the master excitation laser.

Figure 2 illustrates an optical circuit for a wavelength stabilized active mode locked fiber laser in accordance with an embodiment of the present disclosure.

In one embodiment, the MLL as shown in figure 2 comprising at least an optical fiber ring cavity (201) and a pump laser source (202) and communicatively coupled with each other. The pump laser source (202) is a laser diode or a laser diode array for supplying optical energy into the optical ring cavity (201). The optical energy generated by the pump laser source (202) has an approximate wavelength of 980 ran or alternately at 915 ran. The pump laser source is connected to an isolator (204). The optical isolator (204) transfers the pump laser energy into the optical ring cavity (201). The optical isolator (204) also rejects any optical energy reflected back from the optical ring cavity (201) and protects the pump laser source (202).

The optical ring cavity (201) comprising at least an Electro-Optic Modulator (EOM) (206) communicatively connected to an output end of the RF circuit (106) for receiving an electrical signal generated from a predetermined number or a fraction of the master laser optical signals. The EOM (206) receives the electrical signals from the RF circuit (106) and converts the received electrical signal into plurality of optical pulses. The EOM (206) is used as an inter-cavity shutter. In one embodiment, the EOM (206) is a broad bandwidth modulator, with a fast shutter turn-on and shutter turn-off time thus allowing a short on-time. The EOM (206) operating as a shutter that shapes the optical pulse to narrower pulse using the electrical pulses from the RF circuit as the shutter closing triggers in a process called active mode-locking. The narrowest pulses are formed when the EOM (206) is driven at the same time interval as required for the pulses to complete one round trip through the optical ring cavity (201). The EOM (206) operates as an active element of the MLL (108) and controls the loss modulation in the optical fiber ring cavity (201) based on the output of the RF circuit (106). In another embodiment, the EOM (206) is driven by drive electronics which may be any electronic circuit known in the art capable of supplying pulses of sufficient magnitude to activate the EOM (206).

The EOM (206) is coupled to a Wavelength-Division Multiplexer (WDM) (208) and configured to transmit the output optical pulses to the WDM (208). The WDM is a multiplexer that is capable of combining plurality of input signals at different wavelengths into a combined single output signal. In one embodiment, the WDM (208) is coupled to receive input pulses from both the EOM (206) and optical energy from the pump source (202), both having different wavelengths. The output of the WDM (208) is the pump laser energy multiplexed with the circulating optical pulses and communicated to an optical gain fiber.

In one embodiment, the optical gain fibre is an Ytterbium doped fiber (YDF) (210) that is communicatively coupled with the WDM (208) to receive the multiplexed circulating optical pulses and the pump laser from the WDM (208). In another embodiment, the optical gain fiber is any other gain medium available in the art. The YDF (210) is a doped fiber of predetermined length. The predetermined length of the YDF (210) may be varied to adjust the length of the optical ring cavity (201). The YDF (210) amplifies the received circulating optical pulses using energy from the pump laser source, and transmits the amplified pulses to a wavelength selective filter for restricting the amplified pulse to a predetermined wavelength.

In one embodiment, the wavelength selective filter is a Fiber Bragg Grating (FBG) (212) a type of reflector that reflects pulses of particular wavelength and transmits the other pulses. In another embodiment, the FBG (212) is a chirped FBG to generate optical pulses having narrow width. The length of the FBG (212) may also be varied to generate pulses of tuned wavelength. The FBG (212) comprises a predetermined number of ports to reflect pulses of one or more wavelengths.

In one embodiment, the FBG (212) is communicatively coupled to the fiber ring cavity via a circulator (213) comprising three ports such as port 1, port 2 and port 3, disposed at equidistance there between as shown in figure 2. The amplified optical pulse received from the YDF (210) is fed to port 1 of the circulator (213). At the circulator (213) the optical pulse received at port 1 is transmitted to port 2. The FBG (212) connected at the port two reflects the optical pulse back to port 2 of circulator (213). The reflected optical pulse received at port 2 is transmitted to port 3. The final reflected optical pulse from the port 3 is transmitted as an output through an output coupler (214). The final reflected output pulse is a wavelength stabilized optical pulse having a short pulse width in the order of hundred pico-second pulse width and predetermined wavelength such as 1064 ran wavelength. The final output pulses from the optical ring cavity (201) are generated with a repetition rate that is matching with the repetition rate of the master laser pulses and therefore, a wavelength stabilized slave pulse train is generated that is synchronous with the master laser pulses.

The output coupler (214) transmits a predetermined portion of the output pulse and the remaining portion is being fed back as an input to the EOM (206). In one embodiment, the output coupler is a 30% splitter i.e., 30 % of the optical pulse is transmitted as output and the remaining 70% of the optical pulse is fed back as input into the EOM (206).

In one embodiment, the MLL (108) is designed to achieve mode locking at a predetermined frequency for example, 80.1 MHz. In order to achieve locking at the fundamental harmonic, the length of the optical ring cavity (201) may be around 2.5 m having refractive index (n) of 1.5. Instead, the length of the optical ring cavity (201) is controlled so that the round trip time spent by a pulse in the ring exactly matches with a sub-harmonic of the optical signal derived from the master laser source. For example, if the master laser source is providing laser pulses at 80.1 MHz, approximately at every 12.48 ns, the length of the optical ring cavity (201) is controlled so as to match a round trip of 49.92 ns which is exactly four times the fundamental round trip.

Let us consider, the output pulse generated by the MLL (108) is having a predetermined wavelength 'X' nm which is amplified by the optical amplifier (110). The amplified output pulse having 'X' nm wavelength is converted by the SHG crystal (112) into a pulse having 'Υ' nm wavelength. The master laser is operated at a wavelength 'Z' nm such that difference between half the wavelength 'Z/2' nm of the master laser and the output 'Υ' nm wavelength is a predetermined value. For example, in one embodiment, the wavelength stabilized optical pulse generated by MLL (108) has a predetermined wavelength of 1064 nm which is amplified by the optical amplifier and fed to SHG crystal for further processing. The SHG crystal (112) converts the amplified depletion pulse of 1064 nm wavelength into output depletion pulse of 532 nm wavelength such that if the master laser wavelength is 800nm and half the wavelength of the master laser is 400 nm, then the difference between the wavelength of the master laser and the output depletion pulse is maintained to be 132 nm. Figure 3 illustrates an optical amplifier in accordance with an embodiment of the present disclosure.

In one embodiment, the length of the optical cavity (201) is close to but not exactly matched to an integral multiple of the round trip time of an optical pulse in the optical cavity (201). The optical cavity (201) is thus operating slightly detuned and the optical pulses generated at the output of the coupler (214) will have higher pulse widths. In addition, the optical pulses output from the coupler (214) will exhibit small variations in amplitude. These optical pulses are received by the optical amplifier (110). The optical amplifier (110) is operated at maximum amplification, or saturation, such that the output of the optical amplifier has optical pulses of equal amplitude. These are then received communicatively by the second harmonic generation unit.

Figure 4 illustrates a graphical representation of the variation in optical pulse width of the slave laser in accordance with an embodiment of the present disclosure.

As shown in figure 4, the graph represents with frequency of the electrical pulses derived from the master laser along the 'X' axis and represents along the Ύ' axis the optical slave pulse width.

In one embodiment, the electrical signal derived from the master laser pulses, are output from the RF circuit (106) as electrical pulses the effect of which on the slave optical pulse width is illustrated as (402). In another embodiment the electrical signal derived from the master laser pulses and output from the RF circuit (106) are sinusoidal with the same frequency as the repetition rate of the master laser pulses, the effect of which is illustrated as (404). The electrical output from the RF circuit (106) is received communicatively as input to the EOM.

The optical amplifier (110) achieves amplification of the wavelength stabilized laser pulse. In one embodiment, the optical amplifier (110), as shown in Figure 3 is a Master Oscillator Power Amplifier (MOPA). The MOPA may be built using a single stage double clad fiber (DCF) or a large mode area (LMA) fiber. In another embodiment, the optical amplifier may be an optical amplifier known in the art.

The optical amplifier (110) comprising a pump laser source (302), a first isolator (304), a pump coupler (306), an Ytterbium doped fiber (Yb: DCF) (308) and a second isolator (310). The wavelength stabilized depletion pulse laser generated by the MLL (108) is fed as input to the optical amplifier via the first isolator (304). The first isolator (304) receives the stabilized pulse, prevents any leakage back to the MLL (108) and forwards the stabilized pulse to the pump coupler (306).

The pump coupler (306) receives the wavelength stabilized depletion pulse from the first isolator (304) and optical energy signals at 915nm wavelength from the pump laser source (302). The pump coupler (306) couples and feeds the received signals to the Yb: DCF (308). The Yb: DCF (308) is a single stage clad fiber amplifier that amplifies the received signals having different wavelengths and transmits the amplified signal to the second isolator (310). The second isolator (310) prevents reflection or leakage of the amplified signal back into the Yb: DCF (308) and transmits the high powered amplified depletion pulse to the SHG crystal (112).

In one embodiment, the optical amplifier (110) receives the depletion laser pulse of 1064 nm wavelength for example, and amplifies the received pulse laser. The amplified pulse laser of 1064 nm wavelength is then fed to the SHG crystal (112) which converts the depletion laser pulse of 1064 nm wavelength into a depletion pulse laser of 532 nm wavelength, thereby achieving a predetermined wavelength difference of 100 nm between the wavelengths of master or excitation laser and depletion laser pulses.

Figure 5 illustrates a graphical representation of excitation and depletion laser in accordance with an embodiment of the present disclosure.

As shown in figure 5, the graph illustrates wavelength (nm) represented along the 'X' axis with the wavelengths of the excitation spectrum, emission spectrum and depletion laser spectrum shown clearly. In one embodiment, the excitation spectrum (502) illustrates the excitation characteristics of molecules of the imaging samples and the emission spectrum (504) illustrates the emission characteristics of the molecules. The depletion spectrum (506) graphically represents the output from the laser system falling within the emission spectrum (504) where the excitation spectrum (502) is zero. Thus, the depletion spectrum (506) shown at 532 nm wavelength achieves a predetermined wavelength difference of 100 nm between the wavelengths of master or excitation laser and depletion laser pulse which are synchronized with the excitation laser, hence achieving desired aspects to obtain better imaging resolution of the samples.

Alternative to electronic synchronization, optical coupling can also be achieved between the excitation and depletion pulse lasers by seeding a slave laser with the signal from a master laser. When the master laser is pulsed, the slave laser also produces pulses, not necessarily at the same wavelength. The two outputs from the master and slave laser can then be independently tuned or frequency doubled to a suitable visible wavelength.

The present disclosure can be used as an add-on to an existing microscopy imaging system. Further, the present disclosure can also be re-optimized for use in systems such as LIDAR, pump-probe experiments, and for THz generation where there is a need for short high power optical pulses of two or more wavelengths, with high repetition rates and synchronized with each other.

The present disclosure is not to be limited in terms of the particular embodiments described in this application, which are intended as illustrations of various aspects. Many modifications and variations can be made without departing from its spirit and scope, as will be apparent to those skilled in the art. Functionally equivalent methods and devices within the scope of the disclosure, in addition to those enumerated herein, will be apparent to those skilled in the art from the foregoing descriptions. Such modifications and variations are intended to fall within the scope of the appended claims. The present disclosure is to be limited only by the terms of the appended claims, along with the full scope of equivalents to which such claims are entitled. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting.

While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.

Claims

Claims:

1. A mode locked laser (MLL) for generating a wavelength stabilized optical slave pulse train, the MLL comprising:

a pump laser source for generating of optical energy; and

an optical fiber ring cavity to receive the optical energy from the pump laser source, the optical fiber ring cavity comprising:

an electro-optic modulator (EOM) configured to receive electrical signals from a RF circuit and convert the received electrical signals into corresponding slave optical pulses;

an RF circuit configured to receive electrical signals derived from the optical pulses of a master laser through a photodetector;

an optical gain medium, coupled to the EOM, configured to receive and amplify circulating optical pulses and multiplexed with optical energy from a pump laser source; and

a wavelength selective filter, coupled to the optical gain medium, configured to filter the amplified circulating pulses multiplexed to the pump laser source and co-propagating in the optical gain medium, to generate wavelength stabilized slave optical pulses, wherein the generated wavelength stabilized slave pulses are of predetermined wavelength with a repetition rate matching with repetition rate of the received master laser pulses.

2. The MLL as claimed in claim 1, wherein the EOM receives an electrical signal derived from a predetermined fraction of optical signals from the master laser source through a Radio Frequency (RF) circuit.

3. The MLL as claimed in claim 1 , wherein the RF circuit comprising: a photodetector that receives a predetermined fraction of optical signals from the master laser source; and

an electrical circuit that amplifies and filters the received electrical signal from the photodetector; and an electrical circuit that outputs an electrical signal to an electro-optic modulator (EOM) The MLL as claimed in claim 1 , wherein the wavelength selective filter comprising: a fiber Bragg grating (FBG) unit to generate optical pulses of predetermined wavelength; and

a circulator comprising plurality of ports, where each port is configured to receive optical pulses, reflect a pulse at predetermined wavelength from the received optical pulses from the FBG and generate a wavelength stabilized slave optical pulse. The MLL as claimed in claim 1, wherein the optical gain medium receives multiplexed pulses from a wavelength-division multiplexer (WDM) that is coupled with the EOM and the pump laser source. The MLL as claimed in claim 5, wherein the WDM is configured to receive and multiplex the circulating optical pulses from the EOM with optical energy from the pump laser source. The MLL as claimed in claim 6, wherein the circulating optical pulses and pump energy multiplexed by the WDM are having wavelength in the range of 970 to 1070 nm. The MLL as claimed in claim 1, wherein the optical gain medium is an Ytterbium doped fiber (YDF). The MLL as claimed in claim 1, wherein the wavelength stabilized depletion pulse is fed to an output coupler that transmits a fraction of the wavelength stabilized pulse as an output and transmits remaining fraction of the wavelength stabilized pulse as an input to the EOM. The MLL as claimed in claim 1, wherein the predetermined wavelength of the wavelength stabilized depletion pulse is approximately 1064nm.

11. The MLL as claimed in claim 1, wherein the pulse width of wavelength stabilized depletion pulse is approximately 100 ps.

12. The MLL as claimed in claim 1, wherein the optical fiber ring cavity is configured with a predetermined length of approximately 10 m with a round trip time of approximately 50 ns to match an integral multiple of the time interval of 12.5 ns between consecutive master laser pulses.

13. The MLL as claimed in claim 12, operated at the fourth harmonic of its fundamental operating frequency, to match the repetition rate of the master laser pulses

14. A method of generating a wavelength stabilized optical signal by operating a mode- locked laser, the method comprising:

generating, by a pump laser source, optical energy;

receiving, by an electro-optic modulator (EOM), an electrical signal derived from a fraction of master laser pulses and converting into slave pulses of predetermined wavelength having a repetition rate matching with a repetition rate of the received master laser pulses;

multiplexing, by a wavelength division multiplexer, the circulating pulses and the pump laser source, and amplifying in a Ytterbium doped fibre the multiplexed circulating pulses with optical energy from the pump laser source; and

generating, by a wavelength selective filter, a wavelength stabilized optical pulse having a predetermined wavelength by filtering the amplified circulating pulses.

15. The method as claimed in claim 14, wherein a predetermined fraction of the wavelength stabilized pulse is transmitted as an output and the remaining fraction of the wavelength stabilized pulse is fed back as an input to the EOM, by an output coupler of the MLL.

16. A fiber laser system for generating synchronized high power optical pulses, the fiber laser system comprising: a mode-locked laser (MIX) configured to receive a predetermined fraction of master laser pulses having a first predetermined wavelength and generate a wavelength stabilized slave laser pulse;

an optical amplifier communicatively coupled with the MLL, wherein the optical amplifier is configured to receive and amplify the wavelength stabilized optical slave pulse to generate amplified wavelength stabilized optical slave pulse with equal optical amplitudes; and

a second harmonic generation (SHG) crystal, communicatively coupled with the optical amplifier, to convert the amplified optical slave pulses into depletion pulses synchronized with the master laser pulses having a second predetermined wavelength; an acousto-optic modulator, communicatively coupled with the master laser, to convert the master laser pulses to a second predetermined wavelength

wherein the second predetermined wavelength of the depletion pulse exceeds the second predetermined wavelength of the master laser pulse by a predetermined value.

17. The system as claimed in claim 16, wherein the first predetermined wavelength of the master laser is approximately 450 nm.

18. The system as claimed in claim 16, wherein the second predetermined wavelength of the master laser is approximately 532 nm.

19. The system as claimed in claim 16, wherein the optical amplifier is a master oscillator power amplifier using a single stage double clad fiber (DCF).

20. The system as claimed in claim 16, wherein master laser pulses are generated by an excitation laser source.

21. The system as claimed in claim 20, wherein the excitation laser source comprises a dispersive element, configured to tune the master laser pulses for matching with an optimal repetition rate of the depletion pulses.