AU2005309330B2

AU2005309330B2 - Method and apparatus for modifying out of band reflection for a laser element

Info

Publication number: AU2005309330B2
Application number: AU2005309330A
Authority: AU
Inventors: Scott Foster; Alexei Tikhomirov
Original assignee: Commonwealth of Australia
Current assignee: Commonwealth of Australia
Priority date: 2004-11-26
Filing date: 2005-11-25
Publication date: 2011-04-07
Anticipated expiration: 2025-11-25
Also published as: AU2005309330A1

Description

WO 2006/056016 PCT/AU2005/001784 METHOD AND APPARATUS FOR MODIFYING OUT OF BAND REFLECTION FOR A LASER ELEMENT FIELD OF THE INVENTION 5 The present invention relates to lasers. In one particular form the present invention relates to a distributed feedback (DFB) fibre laser having improved characteristics for use in fibre laser arrays. BACKGROUND OF THE INVENTION 10 DFB lasers are a variety of lasers which include one or more Bragg gratings which act as reflection elements within a laser active region. This technique of co-locating the gain medium and the feedback grating is applicable to fibre lasers such as those which employ a gain medium that has been doped with erbium. 15 An example of a prior art DFB fibre laser is illustrated in Figure 1. Fibre laser 100 includes a doped fibre 110 and Bragg grating 120 incorporating a phase discontinuity located in the middle section 130 of grating 120. The Bragg grating is provided by a UV induced periodic spatial variation of the 20 refractive index of the gain medium. Other techniques which provide a Bragg grating structure include periodic modulation of the gain or loss of the active region or potentially the cutting of a periodic pattern of grooves into the cladding of the fibre might also conceivably be used. 25 Fibre laser 100 is activated by optical pumping 140 which involves pumping light having a wavelength that matches with the appropriate absorption band of the active material or gain medium through a passive fibre connected to fibre laser 100. This arrangement of the Bragg grating 120 and gain medium provides optical feedback at approximately the Bragg wavelength A, WO 2006/056016 PCT/AU2005/001784 2 characterised by the relation A. = 2ne A where A is the period of the grating and neff is the effective refractive index of the fibre mode. The grating is characterised by a complex coupling coefficient 5 ic(z) = 7cAn(z)e"'(z)/ A where An is the refractive index modulation and Qp(z) is the phase error associated with the grating and where z is a measure of the longitudinal distance along the fibre. Accordingly the spectral width of the grating reflection is proportional to I'd. 10 As illustrated figuratively in Figure 2, a r phase shift is introduced into the middle section 130 of grating 120. The introduction of this phase shift ensures a lowest threshold, highly confined fundamental laser mode operating at essentially the Bragg wavelength A.. The typical field distribution of such a laser is shown in Figure 3 where it can be seen that the field has a maximum 15 at the location of the phase shift and decays exponentially away from the centre of grating 120. The spatial width of the field distribution depends on I'd and defines the overall device length L which in practice is usually a few centimetres. 20 One of the major applications of a DFB fibre laser is to incorporate a number of fibre lasers into one continuous fibre to form a fibre laser array. Each of the fibre lasers are tuned to operate at slightly different wavelengths A, B 2 etc with the advantage that optical pumping at a single wavelength may be employed to cause each of the DFB fibre laser sections to lase. This provides a 25 means for wave division multiplexing as laser emissions from each fibre laser section travel down the common fibre and may be sampled using interferometric processing downstream.

WO 2006/056016 PCT/AU2005/001784 3 Arrays of DFB fibre lasers of this type have been employed in a number of applications including sensor arrays where the wavelength output of each fibre laser varies according to the local value of a physical characteristic of the environment such as the temperature or level of sound, to uses such as multi 5 wavelength laser sources. Clearly, the ability of each fibre laser section to emit light essentially at the respective Bragg wavelength is critical as each of the wavelengths AB, IB 2 etc. will be tightly spaced due to the finite emission band-width available to the gain medium, which must be similar for each laser due to the requirement that each fibre laser is activated by pump light 10 having the same wavelength. However, DFB fibre lasers have a number of disadvantages which directly affect the performance of fibre laser arrays based on a number of fibre laser sections. Although the Bragg grating is designed to reflect light in only a 15 narrow band about the Bragg wavelength AB and to be essentially transparent outside the band there is in practice out of band reflection associated with the side-lobes of the Bragg grating. The out of band reflection r(v) is characterised by the relationship, L 20 r(v)= - c(z) -exp(-i21rvz)dz 0 where v is the detuning from the Bragg frequency and is defined by v = 2neff for v > IK . When two or more DFB fibre lasers are connected to the same fibre, this out of band reflection results in a fraction of light from a given fibre 25 laser section being reflected by another fibre laser section thereby causing a shift AA from the Bragg wavelength AB for that particular fibre laser section.

4 For distance d between each fibre laser section this wavelength shift is approximated by AA = AI r |e-' sin(2rd /

A

s -,)/ where o, is the phase of the out of band reflection r(v) (i.e. r(v) = Irle"'" ) from the adjacent laser. Accordingly, the laser wavelength will be sensitive to both small changes in distance d between the fibre 5 laser sections and the reflection coefficient r(v) from the adjacent lasers. Clearly, this is undesirable in the example of a sensor array as the intent is to measure changes to the laser wavelength caused by local changes to the Bragg wavelength of the grating of the respective fibre laser section. To address this issue of undesirable wavelength sensitivity, the physical length L of the grating structure 0 can be increased. However, this has the obvious disadvantage of lengthening the fibre laser array where compactness is often a major requirement. In addition where the fibre laser sections are being employed in a sensor array such as an acoustic sensor, lengthening of each fibre laser section implies that a sample is taken from a distributed region as opposed to the fibre laser section acting as a point sensor. Often a sensor design will also require multiple point sensors in close proximity and lengthening the grating 5 structure for each fibre laser section can greatly add to the mechanical constraints in dealing with such a sensor array. It is an object of the invention to provide a DFB laser having improved characteristics that enable the incorporation of these devices into multiple DFB laser arrangements. '0 SUMMARY OF THE INVENTION In a first aspect the present invention accordingly provides a distributed feedback fiber laser element for producing laser light including: an active medium excited by optical pumping means to produce stimulated emission of light; and 25 a Bragg grating structure for providing optical feedback for said active medium in a wavelength band, said Bragg grating structure including a phase transition region providing a change in phase, wherein said change in phase of said phase transition region of the Bragg grating structure is adjusted to continuously change over an extended region of said laser element to reduce out of band reflection associated with side lobes of the Bragg grating structure to reduce a fraction of light reflected by the 30 Bragg grating structure outside of the wavelength band of said laser element. By modifying the out of band characteristics of the laser element the laser element may be customised for incorporation into a system incorporating an array of multiple laser elements.

5 In another form, a maximum phase change AcI of said change in phase is greater than n. In another form, said maximum phase change A$ of said change in phase is determined in part by a length of said extended region. In another form, said maximum phase change AT increases as said length of said extended region increases. In another form, said change in phase is characterized by a function #(z) = f, (z)Al where z is the length along the laser element and f, (z) is a function that varies smoothly from 0 to 1. In another form, said maximum phase change AT is determined by solving the coupled equations for A$ and auxiliary function q(z): Act = ;r + 2 K(z) sin(q(z))dz q(z) = ADf, (z)- 2 JK(z')sin(q(z'))dz' '2 where K(z) is a coupling coefficient of said Bragg grating structure; and z 2 and z 3 define the boundaries of said phase transition region. In another form, said maximum phase change A$ of said smooth change in phase is determined by AO - 2 . K(Z) -sin(a))dz = r

Z

2 where K(z) is a coupling coefficient of said Bragg grating structure; and z 2 and z 3 define the boundaries of said phase transition region. In a second aspect the present invention accordingly provides a method for producing laser light from a distributed feedback fiber laser element, said method including the steps: optically pumping an active medium to produce stimulation emission of laser light; and adjusting a change in phase of a phase transition region continuously over an extended region of said laser element of a Bragg grating structure providing optical feedback for said active medium in a wavelength band to reduce out of band reflection associated with side lobes of the Bragg grating structure to reduce a fraction of light reflected by the Bragg grating structure outside of the wavelength band of said laser element.

6 In a third aspect the present invention accordingly provides a distributed feedback fibre laser element for producing laser light including: an active medium excited by optical pumping means to produce stimulated emission of light; and a Bragg grating structure for providing optical feedback for said active medium, the Bragg grating structure having a complex coupling coefficient K(z) characterised by the relationship K(Z) = ffAn(z)e~"() /A where z is a measure of the longitudinal distance along the distributed feedback fibre laser element having an overall device length L , An(z) is a refractive index modulation of the Bragg grating structure, said Bragg grating structure including a phase transition region providing a change in phase characterised by a ir phase shift introduced into the Bragg grating structure to provide a lowest threshold, highly confined fundamental laser mode operating at the Bragg wavelength A, wherein said change in phase (o(z) of said phase transition region is adjusted to continuously change over an extended region of the distributed feedback fibre laser element with a maximum phase change A'D greater than n to reduce out of band reflection r(v) of the Bragg grating structure of said distributed feedback fibre laser element, the out of band reflection r(v) of the Bragg grating structure characterised by the relationship: r(v)=- JK(z) 'exp(-i2r1Vz)dz 0 where v is the detuning from the Bragg frequency defined by v = 2 n, - for v > K and neff is the effective refractive index of the fibre mode. In another form, p(z) is characterised by a continuous phase transition #(z)= f, (z)AI where f, (z) is a function with the boundary conditions f, (z 2 ) = 0 and fl (z 3 ) = I where z 2 and z 3 define the boundaries of said phase transition region. In a fourth aspect the present invention accordingly provides a method for producing laser light from a distributed feedback fibre laser element, said method including the steps: optically pumping an active medium to produce stimulation emission of light; and adjusting a change in phase of a phase transition region of a Bragg grating structure, the phase transition region providing a change in phase (p(z), the Bragg grating structure providing optical feedback for said active medium and having a complex coupling coefficient K(z) characterised by the relationship K(z) = rAn(z)e~*(z) /A where z is a measure of the longitudinal distance along the 7 distributed feedback fibre laser element having an overall device length L , An(z) is a refractive index modulation of the Bragg grating structure and a / phase shift is initially introduced into the Bragg grating structure to provide a lowest threshold, highly confined fundamental laser mode operating at the Bragg wavelength /,, wherein adjusting the change in phase of the phase transition region includes adjusting the change in phase qp(z) of said phase transition region to continuously change over an extended region of the distributed feedback fibre laser element with a maximum phase change AcI greater than r to reduce out of band reflection r(v) of the Bragg grating structure of said distributed feedback fibre laser element, the out of band reflection r(v) of the Bragg grating structure characterised by the relationship: L r(v)= - K(z) -exp(-i2;nvz)dz 0 where v is the detuning from the Bragg frequency defined by v = 2 nf[ - for v > IK and ne, is the effective refractive index of the fibre mode. In another form, rp(z) is characterized by a continuous phase transition #(z) = f (z)At where f, (z) is a function with the boundary conditions f (z 2 ) =0 and f (z 3 ) =1 where z 2 and z 3 define the boundaries of said phase transition region. BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of the present invention will be discussed with reference to the accompanying drawings wherein: FIGURE I is a figurative representation of a distributed feedback (DFB) fibre laser (FL) as known in the prior art; FIGURE 2 is a depiction of the r phase shift introduced in the central lasing region of the DFB FL illustrated in Figure 1; FIGURE 3 is an example plot of the field distribution of the DFB FL as illustrated in Figure 1; FIGURE 4 is a plot of the amplitude and phase apodisation profiles according to a preferred embodiment of the present invention; FIGURE 5 is a plot of the field distribution of an apodised DFB FL laser when modified according to the apodisation profiles illustrated in Figure 4 compared to the field distribution of standard DFB FL; and FIGURE 6 is a comparison plot of the measured spectral reflection curve of a non-apodised and apodised DFB FL modified according to the apodisation profiles illustrated in Figure 4.

7a DESCRIPTION OF PREFERRED EMBODIMENT Referring now to Figure 4, there is shown a modification of a phase transition region of a distributed feedback fibre laser (hereinafter a DFB FL) according to a preferred embodiment of the present invention. Whilst in this preferred embodiment the phase transition region of a DFB FL has been modified, it would be apparent to those skilled in the art that the invention may be equally applied to modify the out of band reflection characteristics of other varieties of Bragg grating lasers which incorporate central phase transition regions where the phase rapidly varies.

WO 2006/056016 PCT/AU2005/001784 8 DFB FL is assumed to be of length L and is divided into five regions corresponding to first region ranging from 0 < z < zj, second region zi <z<z 2 , third region z 2 < z < z 3 , fourth region z 3 <z<z 4 and fifth region z4 < z < L where z measures longitudinal extent along the fibre. Apodisation 5 is applied to both the amplitude and phase of the grating coupling coefficient W(z). Phase apodisation is applied to the third region which would typically be a step function in a prior art DFB FL such as that illustrated in Figure 1. 10 According to this preferred embodiment of the present invention, a continuous phase transition #(z) is introduced which is defined by the relationship #(z) = f, (z)AD with the boundary conditions f 1 (z 2 )=0 and f 1 (Z3) =1. 15 In this embodiment f, (z) is chosen to achieve best reflection suppression on the defined transmission length z 3 - z 2 and is defined by fA(z) = Cos" -_ _ 2(Z3= -Z2) 2(Z3 -Z2) for n = 2,4, etc. Depending on the apodisation requirements, other suitably defined smooth transition functions which vary from 0 to 1 and whose 20 derivatives vanish at the relevant boundaries may be employed. The value for the constant A(D is determined by solving numerically the following coupled equations thereby determining the value of A1 that ensures optimal single mode performance at the Bragg frequency. 25 AcI = +2 r(z) sin(q(z))dz

Z

2 9 q(z) = Ac!f, (z) - 2 Jic(z')sin(q(z'))dz' For the derivation of these equations see in particular Equation 19 as described in the article entitled "Experimental and Theoretical Characterisation of the Mode Profile of Single-Mode DFB Fiber Lasers" (IEEE Journal of Quantum Electronics, Vol. 41, No. 6, June 2005) which is herein incorporated by 5 reference in its entirety. These coupled equations are then solved iteratively for Ac for reasonable choices of f 1 (z) such as that described above. In certain cases where Ac! is close to if, then the first order approximation 0 A!) - 2. J(z) -sin(A!f, (z))dz = ;r may be adequate. Whilst in this preferred embodiment Ac! , has been calculated according to the above relationships, clearly other values may be calculated and used according to the exact tuning requirements of the DFB FL being contemplated. 5 According to these calculations, the phase shift step value or maximum phase change Ac! will always be greater than ir. As would be appreciated by those skilled in the art, for prior art DFB FLs the optimal condition for single mode performance whereby the optimum amount of energy is confined in one mode has always incorporated a phase shift step value or maximum phase change of 7f. 20 Additionally the amplitude Ic(z) of grating coupling coefficient may also be modified. Referring again to Figure 4, |1c(z) is modified according to the WO 2006/056016 PCT/AU2005/001784 10 relationship li(z) = f 2 (z) .I where f2 (0) =f 2 (L) =0 and f 2 (z)=1 for z 1 <z <z 4 . For first and fifth regions f 2 (z) is defined in a similar manner to f,(z) . Whilst amplitude apodisation of this nature is known in the prior art it does not in of itself successfully address issues with out of band reflection 5 as highlighted previously. However, it may be employed in addition to phase apodisation according to the present invention to further reduce the effects of side lobes thereby resulting in minimised out of band reflection in a fibre laser section. 10 Referring now to Figure 5, a calculated curve for the field distribution of a DFB FL which has been apodised according to the present invention is compared with the field distribution of a corresponding standard DFB FL. For this embodiment, the region z 3 z 2 corresponds to 0.2 L. Although the phase shift region has now extended in size to occupy approximately 20% of the 15 device length, the associated increase in the laser mode width and hence overall device length is only 4% and as such only represents a very small increase. Referring now to Figure 6, plot B depicts the measured spectral reflection 20 curve from a DFB FL apodised according to the present invention and employing parameters co = 1.9 cmr 1 and ACD = 4.5 radians thereby illustrating that reflection values of less than -50 dB are achievable. For comparison, plot A depicts the spectral reflection curve for a non-apodised laser of the prior art. 25 Accordingly, this invention makes it possible to achieve very low out of band reflectivity without having to substantially increase the total device length thereby making DFB FLs adopting this invention most suitable for WO 2006/056016 PCT/AU2005/001784 11 incorporation in linear multiplexed fibre laser arrays. The invention partly resides in realising that scattering from the normal discrete g phase shift employed in standard DFB FL contributes substantially to the spectral reflection curve. According to the present invention, adjusting and modifying 5 the shape and/or associated magnitude of this phase shift is important when attempting to substantially reduce the out of band spectral reflection. Although a preferred embodiment of the present invention has been described in the foregoing detailed description, it will be understood that the 10 invention is not limited to the embodiment disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope of the invention as set forth and defined by the following claims.

Claims

3. A laser element for producing laser light as claimed in claim 2, wherein said maximum phase change A'D of said change in phase is determined in part by a length of said extended region.
4. A laser element for producing laser light as claimed in claim 3, wherein said maximum phase 5 change AcI increases as said length of said extended region increases.
5. A laser element for producing laser light as claimed in any one of claims I to 4, wherein said change in phase is characterized by a function #(z) = f, (z)AI where z is the length along the laser element and f, (z) is a function that varies smoothly from 0 to 1.
6. A laser element for producing laser light as claimed in claim 5, wherein said maximum phase 20 change Ac! is determined by solving the coupled equations for AO and auxiliary function q(z): Act = ir + 2 fi(z) sin(q(z))dz q(z) = ADf, (z) - 2 JK(z')sin(q(z'))dz' where K(z) is a coupling coefficient of said Bragg grating structure; and z 2 and z 3 define the boundaries of said phase transition region. 25 7. A laser element for producing laser light as claimed in claim 6, wherein said maximum phase change AQD of said smooth change in phase is determined by 13 A(I -2. -f(z) . sin(ADf, (z))dz = ; Z 2 where K(z) is a coupling coefficient of said Bragg grating structure; and z 2 and z 3 define the boundaries of said phase transition region. 5 8. A method for producing laser light from a distributed feedback fiber laser element, said method including the steps: optically pumping an active medium to produce stimulation emission of laser light; and adjusting a change in phase of a phase transition region continuously over an extended region of said laser element of a Bragg grating structure providing optical feedback for said active medium in a 0 wavelength band to reduce out of band reflection associated with side lobes of the Bragg grating structure to reduce a fraction of light reflected by the Bragg grating structure outside of the wavelength band of said laser element.
9. A method for producing laser light from a laser element as claimed in claim 8, wherein a maximum phase change A$ of said change in phase is greater than 1r. 5 10. A method for producing laser light from a laser element as claimed in claim 9, wherein said maximum phase change AO of said change in phase is determined in part by a length of said extended region.
11. A method for producing laser light from a laser element as claimed in claim 10, wherein said maximum phase change A$ increases as said length of said extended region increases. 20 12. A method for producing laser light from a laser element as claimed in any one of claims 8 to 11, wherein said change in phase is characterized by a function #(z) = f, (z)AI where z is the length along the laser element and f, (z) is a function that varies smoothly from 0 to 1.
13. A method for producing laser light from a laser element as claimed in claim 12, said method further including the step of determining said maximum phase change AcD by solving the coupled 25 equations for AQ and auxiliary function q(z): AoD = 7r + 2 K(z)sin(q(z))dz 14 q(z) = AIfI (z) - 2 JK(z') sin(q(z'))dz' Z 2 where K(z) is a coupling coefficient of said Bragg grating structure; and z 2 and z 3 define the boundaries of said phase transition region.
14. A method for producing laser light from a laser element as claimed in claim 13, said method 5 further including the step of determining said maximum phase change AcD of said smooth change in phase by solving At, - 2 - JK(z) -sin(AIf, (z))dz = ir Z 2 where K(z) is a coupling coefficient of said Bragg grating structure; and 0 z 2 and z 3 define the boundaries of said phase transition region.
15. A distributed feedback fibre laser element for producing laser light including: an active medium excited by optical pumping means to produce stimulated emission of light; and a Bragg grating structure for providing optical feedback for said active medium, the Bragg grating structure having a complex coupling coefficient K(Z) characterised by the relationship 5 K(Z) = iAn(z)e (z) /A where z is a measure of the longitudinal distance along the distributed feedback fibre laser element having an overall device length L, An(z) is a refractive index modulation of the Bragg grating structure, said Bragg grating structure including a phase transition region providing a change in phase characterised by a if phase shift introduced into the Bragg grating structure to provide a lowest threshold, highly confined fundamental laser mode operating at the Bragg wavelength 2A, 20 wherein said change in phase qp(z) of said phase transition region is adjusted to continuously change over an extended region of the distributed feedback fibre laser element with a maximum phase change A'I greater than ir to reduce out of band reflection r(v) of the Bragg grating structure of said distributed feedback fibre laser element, the out of band reflection r(v) of the Bragg grating structure characterised by the relationship: 25 r(v)=- K(z) -exp(-i2ifvz)dz 0 15 where v is the detuning from the Bragg frequency defined by v = 2 ng - for v > IKI and nff is the effective refractive index of the fibre mode.
16. A laser element for producing laser light as claimed in claim 15, wherein (p(z) is characterized by a continuous phase transition #(z) = f, (z)Ac! where f, (z) is a function with the boundary 5 conditions fl (z 2 ) = 0 and f (z3)=1 where z 2 and z 3 define the boundaries of said phase transition region.
17. A method for producing laser light from a distributed feedback fibre laser element, said method including the steps: optically pumping an active medium to produce stimulation emission of light; and 0 adjusting a change in phase of a phase transition region of a Bragg grating structure, the phase transition region providing a change in phase p(z), the Bragg grating structure providing optical feedback for said active medium and having a complex coupling coefficient K(z) characterised by the relationship ic(z) = 7rAn(z)e- Iz) /A where z is a measure of the longitudinal distance along the distributed feedback fibre laser element having an overall device length L , An(z) is a refractive index 5 modulation of the Bragg grating structure and a ir phase shift is initially introduced into the Bragg grating structure to provide a lowest threshold, highly confined fundamental laser mode operating at the Bragg wavelength A., wherein adjusting the change in phase of the phase transition region includes adjusting the change in phase p(z) of said phase transition region to continuously change over an extended region of the distributed feedback fibre laser element with a maximum phase change Aj 20 greater than z to reduce out of band reflection r(v) of the Bragg grating structure of said distributed feedback fibre laser element, the out of band reflection r(v) of the Bragg grating structure characterised by the relationship: L r(v) = - K(z) -exp(-i2lrv)dz 0 where v is the detuning from the Bragg frequency defined by v = 2 neff - for v> I>I 25 and neff is the effective refractive index of the fibre mode. 16
18. A method for producing laser light from a laser element as claimed in claim 17, wherein (p(z) is characterized by a continuous phase transition #(z) = f, (z)AD where f, (z) is a function with the boundary conditions f (z 2 ) =0 and f (z 3 ) 1 where z 2 and z 3 define the boundaries of said phase transition region. 5 19. A distributed feedback fibre laser substantially in accordance with any one of the embodiments of the invention described herein and illustrated in the accompanying drawings.
20. A method substantially in accordance with any one of the embodiments of the invention described herein and illustrated in the accompanying drawings.