Single-mode, tunable, 980 nm double-clad Ytterbium-doped MMI-based fiber laser

C. Calles-Arriaga1*, R. Selvas-Aguilar1, A. Castillo-Guzman1,

D. A. May-Arrioja2, G. Anzueto-Sanchez3

1Facultad de Ciencias Físico Matemáticas, UANL, Cd. Universitaria, CP 66450, Nuevo León, México

2Photonics and Optical Physics Laboratory, Optics Dept., INAOE, Tonantzintla, CP 72000, Puebla, México

3Universidad Politécnica de Chiapas, Tuxtla Gutierrez, CP 29082, Chiapas, México *E-mail: charlykov@gmail.com

ABSTRACT We report for the first time, a single mode, tunable, double-clad ytterbium-fiber (YDF) laser emitting in a

wavelength range between 976 and 985 nm that operates using the re-imaging effect that occurs in multimode interference (MMI) devices. The system consists of an YDF with bare fiber cleaved ends. The forward end of this fiber is fusion spliced to a piece of 3 m of Samarium-doped- single-mode fiber with absorption measured at 980 nm of 0.3 dB/m, and at 1030 nm of 6 dB/m. The other end of the Sm+3 doped single-mode fiber is spliced to a 16.2 mm long multimode fiber (MMF) in order to induce the MMI self-imaging effect. From simulations, we found that, at this particular length, for the MMF, the light exiting will exhibit a maximum transmission for the 980 nm wavelength, while keeping a minimum for the 1030 nm wavelength. Near to the MMF facet, at a distance between 0 and 100 µm, we place a dichroic mirror which also helps in the selection of the wavelength emission. We calculated that 10 dB gain generated at 980 nm is enough to build up a laser since the total round-trip cavity losses are estimated to be 8.8 dB, whereas for the unwanted 1030nm get more than 60dB insertion loss in this setup. At the end, there is more than 1 dB for the effective gain at the preferred wavelength emission range which is enough to promote lasing at around 980 nm. Keywords: fiber laser, multimode interference effect.

1. INTRODUCTION The last years have witnessed an intense interest and activity in the field of fiber lasers operating at a wavelength of

980 nm1-2. The main motivation is its application as an all-cost effective pump source for EDFA and telecomm subsystems. The advantage of this particular pump wavelength is that the 4I13/2 level is the only band of the erbium that is completely free from excited-state absorption. Moreover, when frequency-doubled to 490nm YDFL´s are the replacement candidates for argon ion lasers or sapphire bulk lasers for using in instrumentation, analytical and bio-medical applications such as flow cytometry or Raman spectroscopy. In addition, 980 nm laser emission is suitable for neurosurgical procedures since it provides a better absorption of water ions compared to the popular Nd:YAG at 1064 nm3. Furthermore, their inherent and outstanding characteristics like high efficiency, small active volume, high output power, and a well controlled beam shape offers a number of important advantages over conventional bulk laser.

Laser action in ytterbium-doped fiber lasers normally occurs at 1.03 µm (four-level transition) and currently it is easy to find in the literature fiber lasers with output power beyond the KW level at that particular wavelength4-7. On the other hand, ytterbium laser also emits on the three level transitions around 980 nm. In fact, laser action at 980 nm was early demonstrated with core-pumping scheme8. These laser sources were reliable and efficient, but limited in power to 1W or less. However, because this is a three-level transition (ground state absorption occurs), scaling to the high power regime with cladding pumping requires a careful and precise design of the double-clad fiber structure. Here, the unwanted quasi 4-L peak emission at 1030nm wavelength is the main cause of this problem, and secondly, the high

Photonic Fiber and Crystal Devices: Advances in Materials andInnovations in Device Applications II, edited by Shizhuo Yin, Ruyan Guo,

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pump threshold at 980 nm obstructs the progress in this emission. Thus, if the pump coupling efficiency could be improved, or the threshold lowered, significant higher power at 980 nm should be achievable.

A good approach was the jacketed-air clad (JAC) fiber which has inherent a higher NA and a small area for the inner cladding. Technically, this fiber offers a good solution to the outlined problems and has been used to demonstrate 980nm sources9. However, the main drawback is related to the complexity of the whole device, which makes this attractive source a very expensive one. In addition, the pump brightness also played an important role for achieving the 980 nm laser emission. The situation gets worse when demonstrating a tunable 980 nm laser, since the system becomes very complex and instable as a result of the bulk tuning element.

Besides the JAC fibers, there were also other techniques employed to filtering out longer wavelengths such as the demonstrated by the W-type fibers10, the depressed-clad hollow fibers, the ring-doping fibers, all of them allowing to explore this important emission wavelength with the Yb3+-ion by suppressing the unwanted 1030 nm emission11.

In recent works, we have demonstrated wavelength tuning for fiber lasers based on the re-imaging process that results from multimode-interference (MMI) effects that take place in multimode fiber waveguides12. In those cases, the wavelength tuning devices used a few cm long multimode fibers with one end spliced to a single mode fiber, while the other end was physically separated from a reflected mirror. By varying this separation, it is possible to tune to the desired lasing wavelength. Using these approaches a tuning range up to 12 nm were measured in an YDFL at emission around 1.06µm, with the tuning range limited primarily by the spectral response of the mirror, and the mechanical alignment of the MMF with the mirror arrangement.

In the MMI effect, the guided modes interferes constructively producing similar images at periodic lengths13. Under this mechanism, images reproduce at lengths of:

πpLLMMI = , (1)

where p is an even or odd integer, and Lπ is the beat length and is defined by:

10 ββπ

π −=L , (2)

where β0 and β1 represent the propagation constants of the two lower order modes in the multimode guide.

Simulations of the MMI effect are based on the Beam Propagation Method (BPM). This method allows the wave equation to be written in the form for monochromatic waves:

( ) 0,, 22

2

2

2

2

2

=+∂∂

+∂∂

+∂∂ φφφφ zyxk

zyx (3)

Considering that the main variation in the field φ is the phase variation, it is introduced a slowly varying field u:

( ) ( ) zkiezyxuzyx ,,,, =φ

Introducing the above expression into the Helmholtz equation yield to the following equation for the slowly varying field:

(4)

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( ) 02 222

2

2

2

2

2

=−+∂∂

+∂∂

+∂∂

+∂∂ ukk

yu

xu

zuki

zu

It is assumed that the variation of u with z is enough slow that the first term in the equation 3 can be neglected. With

this assumption, the equation results:

( ) ⎟⎟⎠

⎞⎜⎜⎝

⎛−+

∂∂

+∂∂

=∂∂ ukk

yu

xu

ki

zu 22

2

2

2

2

2

Equation 6 is the basic BPM equation in three dimensions.

Here, we report for the first time, the numerical demonstration for, a single mode, tunable, double clad Yb3+-fiber laser emitting in a wavelength range between 976 and 985 nm emissions with a simple configuration that includes this novel tuning mechanism. By applying MMI re-imaging theory to optical fibers, a variable filter mechanism is demonstrated. It is also numerically determined that it reduces the 1030 nm fedback signal while is almost transparent for the 980 nm range. Therefore, this filter mechanism not only blocks the unwanted emission, but is also useful for making a tunable fiber laser. In order to guarantee that the 1030 nm emission is completely blocked, we also used a piece of Samarium-doped optical fiber as an unwanted wavelength pre-attenuator mechanism. This fiber is fusion spliced to the double-clad ytterbium-doped gain fiber and thus up to 36 dB of extra insertion loss at 1030 nm is obtained, whereas only 0.3 dB at 980 nm was introduced to the whole system by this samarium fiber. Moreover, by using spectroscopic data of the Yb3+-ion fiber, it is numerically calculated lasing emission spectra with a tuning range that goes from 976 through 985nm.

2. NUMERICAL ANALISIS

We based our gain simulations with an analysis of the relation between the 980 nm gain, the operating pump absorption, and the undesired quasi 4L gain at 1030 nm underlining the problem of unwanted 1030 nm emission. Then, the analysis requires a pump wavelength of 915 nm and signal wavelength of 975 nm, and the undesired gain at 1030 nm to be evaluated with Equation 714. For cross-sections in germanosilicate-based ytterbium-fiber, it becomes:

915915

9751030 72.025.0 op

S

GG α⎟⎟⎠

⎞⎜⎜⎝

⎛ΓΓ

+= , (7)

where G1030 is the gain at 1030 nm, G975 is the gain at 975 nm, αop

915 is the operating pump absorption at 915 nm, Γ915 is the overlap between the Yb3+-doped region and the pump beam at 915 nm, Γs is the corresponding overlap with the signal mode at 915 nm or 1030 nm, and as a consequence from the spectroscopy is understood that the overlap ratio Γs/Γ915 must be smaller than a traditional double-clad fibers in order to achieve a sufficient pump absorption. From simple estimations one can infer that an efficient 980 nm YDFL with a 50 µm inner cladding will have over 1 W of pumping threshold, which is way too high. The characteristics of the jacketed air clad fiber employed in this case are shown in table I.

Table I. Characteristics of the YDF

Fiber HD709-JAC Dopants Yb3+/Ge Concentration 3000 ppm Type Double-cladding Outer cladding 100 µm Inner cladding 27 µm Core diameter 8 µm

(5)

(6)

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The jacketed air clad fiber has several advantages over traditional double-clad fibers, e.g., it provides a higher

numerical aperture which leads to higher pump absorption and higher population inversion; moreover, it is capable of achieving high efficiency and low background loss for short segments of fiber; additionally, this special fiber as halve the diameter, has lower threshold than traditional double-clad fiber.

According to our estimation, it corresponds to a threshold below 300 mW with a pump absorption of 5 dB, and a

conversion efficiency of 40%, which we also estimated it by using a numerical fiber laser simulator which is spectrally, longitudinally and transversally resolved.

3. EXPERIMENTAL SET-UP

Typically a JAC-YDFL with 5 dB absorption at 915 nm and 10 dB gain in the range of 975 nm to 985 nm should

have a 1030 nm unwanted gain ranging from 32 to 45 dB. This unwanted gain may be easily removed with the scheme proposed in this work (See Fig. 1). The system consists of a 30 cm of YDF with bare fiber cleaved ends. The forward end of this fiber is spliced to a piece of 3 m of Sm+3 doped- single-mode fiber. This Sm+3-fiber has absorption measured at 980 nm of 0.3 dB/m, and at 1030 nm of 6 dB/m as shown in Fig 2. The other end of the Sm+3 doped single-mode fiber is fusion spliced to a 16.2 mm long MMF in order to induce the MMI self-imaging effect. Near to the MMF facet, at a distance (D) between 0 and 100 µm, we place a dichroic mirror (HR@915-980nm, HT@1010-1100nm, at 0o degrees) that also helps in the selection of the wavelength emission.

Fig. 1. Experimental set-up of the DCYD laser based on MMI effect.

Round trip losses are estimated by equation 8 as follows:

( ) ⎥⎦

⎤⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛⋅

⋅⋅=21

1ln21log10

RReLC , (8)

The result of this 10 dB gain generated at around 980 nm is enough to build up a laser since the cavity losses,

considering the Fresnel reflection in one end (R = 4%) and a totally reflection mirror at the other end (R = 100%), are estimated to be 7 dB. Moreover, the selected length of the Sm3+doped fiber segment should be efficiently to reduce the gain at 1030 but, at the same time, serving to prevent excessive losses at the desired signal gain. In this case, we considered a 3 m length Sm+3 fiber which results in 18 dB for the unwanted wavelength whereas only 0.9 dB for the 980 nm. The total losses are given by equation 9 as follows:

LT980 = LC + LS980 + LS , (9)

Where LS980 represents absorption losses at 980 nm in the Sm3+ doped fiber, and LS corresponds to the splice losses.

Here, we are considering very low fusion losses, hence LS ≈ 0.01. Thus, overall losses in this scheme are roughly 8.8 dB. At the end, the calculation reveals that there is more than 1 dB of effective gain at the preferred wavelength emission range which is enough to promote lasing at 980 nm.

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Abs

orpt

ion

coef

fice

hit (

dO/m

i

3

120

—a— ID dB gain

NEC 800CO0

60

C

960 965 970 975 980 985

Wavelength (nm)

Previous results from the JAC-fiber (3000 ppm of Yb3+-ion, 8 µm core diameter, clad to core area ratio ∼10) , show

that for a similar overall cavity and pump absorption, there is a gain of 1.37 dB/mW. With this considerably high value, it is relatively easy to achieve laser action.

Fig. 2. Absorption spectra of the Sm3+ doped fiber.

4. RESULTS

Fig. 3 shows numerical modeling of the unwanted gain of 1030 nm for the JAC-YDFL. As can be seen from Fig. 3, the 10 dB gain at 980nm for 5 dB pump absorption results in 32 dB of 1030 nm unwanted gain. There is also interesting that from the range of 976 to 985nm, the unwanted gain is less than 45dB, which means that the setup can work and allow lasing at those particular wavelengths.

Fig. 3 – Unwanted gain at 1030 nm.

Figure 4 shows the results from the numerical simulations using the finite difference beam propagation method (FD-

BPM)) of the beam propagation along a MMF with a core diameter of 105 µm (NA=0.22). As can be seen, the first re-imaging point for the signal at 980 nm occurs at a distance of 16.2 mm, which is the position where the intensity distribution converges. Although, it was not plotted the numerical results for the wavelength ranges from 976 nm through 985nm, it is clear that similar behaviour might be expected, based in that other experimental results at the wavelength range of 1030 nm shows its potential as a tuneable mechanism for saturable fibre gain laser.

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Z (flm)0 3000 6000 9000 12900, 15900 18000

II. .... , •1s S '.lS •e

I- . ..

C _______— — I • I I

030ff,

U,-0--

•______________ _____- I

II [

Fig. 4. FD-BMP along a piece of 105µm MMF at a single wavelength of 980nm at a particular distance for the MMF.

This can also be observed in the monitor value graph, which also shows propagated intensity for 1030 nm (Fig. 5). It is clear that if we cleave the fibre facet of that MMF at this particular length, the light exiting the fiber will exhibit a maximum transmission for the 980 nm wavelength, while keeping a minimum transmission for the 1030 nm unwanted wavelength, that behaviour helps in the required reduction for the 1030 nm gain emission. A simple estimation reveals that up to 20 dB contrast can produce between both wavelengths.

Fig. 5. Transmission monitor for the signal at 980nm (darks line) and 1030 nm (light line) at a particular distance.

5. CONCLUSIONS We model a high gain 980 nm tunable YDFL using a scheme based in the MMI self-imaging effect and the

absorption from the Sm+3 fiber. The YDF is a jacketed air cladding with 5 dB absorption at 915 nm and 10 dB gain in the range of 976 nm to 985 nm and have a 1030 nm unwanted gain of 32 dB. To reduce the unwanted gain, we suppress with the help of the following elements, the forward end of the Yb-fiber is fusion spliced to a piece of 3 m of Sm+3 doped- single-mode fiber. This Sm+3-fiber has absorption at 980 nm of 0.9 dB and losses at 1030 nm of 18 dB. The other end of the Sm+3 doped single-mode fiber was spliced to a 16.2 mm long MMF in order to induce the MMI self-imaging effect, which also induces losses at 1030nm and it is calculated to be more than 20dB. Finally, a dichroic mirror also help as discriminator since it has a HT at the unwanted 1030 nm emission and HR for the desired 980 nm.

Our results shown elimination of the predominant 1030 nm wavelength emission and enhance the 980 nm emission

at a distance of 16.2 mm. Furthermore, using the spectroscopic characteristics of the JAC-fiber we were able to estimate the tuning range which corresponds to a range between 976 and 986 nm.

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ACKNOWLEDGMENTS

This work was supported by the National Mexican Council of Science and Technology CONACYT under grant J1-

61085 and also supported by the program of Nuevos-PTC-PROMEP under grant 2523 from the Mexican Secretariat of Public Education SEP. Romeo Selvas also thanks to the HPFL at the ORC of the Southampton University for the donation of the JAC Yb-Fiber (HD709-JAC).

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