In-situ Tapering of Chalcogenide Fiber for Mid-infrared Supercontinuum Generation

Summary

We describe a method for in-situ tapering of As₂S₃ fibers to achieve efficient mid-infrared supercontinuum generation. By tapering while monitoring the supercontinuum’s spectrum, the spectral width can be maximized for a fiber taper. In-situ fiber tapering can be applied to optimize the performance of other fiber-based devices.

Abstract

Supercontinuum generation (SCG) in a tapered chalcogenide fiber is desirable for broadening mid-infrared (or mid-IR, roughly the 2-20 μm wavelength range) frequency combs^{1, 2} for applications such as molecular fingerprinting, ³ trace gas detection, ⁴ laser-driven particle acceleration, ⁵ and x-ray production via high harmonic generation. ⁶ Achieving efficient SCG in a tapered optical fiber requires precise control of the group velocity dispersion (GVD) and the temporal properties of the optical pulses at the beginning of the fiber, ⁷ which depend strongly on the geometry of the taper. ⁸ Due to variations in the tapering setup and procedure for successive SCG experiments-such as fiber length, tapering environment temperature, or power coupled into the fiber, in-situ spectral monitoring of the SCG is necessary to optimize the output spectrum for a single experiment.

In-situ fiber tapering for SCG consists of coupling the pump source through the fiber to be tapered to a spectral measurement device. The fiber is then tapered while the spectral measurement signal is observed in real-time. When the signal reaches its peak, the tapering is stopped. The in-situ tapering procedure allows for generation of a stable, octave-spanning, mid-IR frequency comb from the sub harmonic of a commercially available near-IR frequency comb. ⁹ This method lowers cost due to the reduction in time and materials required to fabricate an optimal taper with a waist length of only 2 mm.

The in-situ tapering technique can be extended to optimizing microstructured optical fiber (MOF) for SCG¹⁰ or tuning of the passband of MOFs, ¹¹ optimizing tapered fiber pairs for fused fiber couplers¹² and wavelength division multiplexers (WDMs), ¹³ or modifying dispersion compensation for compression or stretching of optical pulses.^14-16

Introduction

After being first produced in the visible wavelength range^1,7 SCG sources have shifted towards the mid-IR, largely driven by applications in spectroscopy.^{3, 4} Chalcogenide fibers, which include sulfides, selenides, and tellurides, have been a popular material for the mid-IR due to their low propagation loss and high nonlinearity,¹⁸ less than 100 dB/km¹⁹ and ~200 times that of silica for As₂S₃,²⁰ respectively. However, the zero GVD wavelength of most chalcogenides is located in the mid-IR, beyond the center wavelength of the majority of available ultrafast pump sources, making SCG c....

Protocol

1. Tapering Setup Fabrication (See Assembled Setup in Figure 1)

1. Secure the motorized linear stages on the breadboard (roughly centered) so that the stages are in contact and will translate towards and away from one another
2. Prepare and place the fiber mounts
   1. Attach two optical posts to the motorized linear stage plates (one each) using the holes closest to each other.
   2. Attach the bare optical fiber mounts to the tops of the posts. Make sure that the v-grooves for the fiber are aligned. (Note: The height of the bare optical fiber mounts will roughly be the beam height of the system. Choose the future post and pedestal heights ....

Results

After successful completion of the in-situ tapering procedure, the pump spectrum broadened to cover from 2.2 to 5 μm (at ~40 dB below the peak), as seen in Figure 3. The pump pulse energy in the As₂S₃ fiber was ~250 pJ with an initial pulse length under 100 fsec. The short length of the tapered waist, ~2.1 mm, allows for generation of a broadband, coherent SC. This preserves the frequency comb properties of the pump source. More information about the frequency comb and othe.......

Discussion

We have demonstrated a novel fiber tapering procedure and verified its validity by performing SCG in the mid-IR. To the best of our knowledge, the alternative method for this application is based on determining the fiber pulling length required to create a tapered fiber diameter that adds enough waveguide dispersion to optimize SCG in the fiber taper through calculation; however, since the pulling length needed to maximize the spectral broadening for a specific length of fiber varies for each experiment, this calc.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Motorized Linear Stages	Newport	MFA-PPD	Available from other vendors.
Motorized Stage Controller	Newport	ESP301	Available from other vendors.
Aluminum Block	Any vendor.		Dimensions will vary depending on desired taper length.
RTD Sensor	Omega	1PT100GX1510
Cartridge Heaters	Omega	CSS-01115/120V
Temperature Controller	Omega	CSC32
Input Coupling Linear Translation Stage	CVI	07TXS224	Available from other vendors.
Output Coupling Linear Translation Stage	Newport	422-1S	Available from other vendors.
XYZ Linear Translation Stage	Newport	461	Available from other vendors.
Assorted posts, optics mounts, bases, and forks	Any vendor.
Optical Breadboard	Thorlabs	MB12	Available from other vendors.
Input Coupling ZnSe Lens	Thorlabs	AL72512-E	Available from other vendors. Input coupling focal length depends on pump source and fiber mode field diameter.
Output Coupling ZnSe Lens	Edmund Optics	NT62-961	Available from other vendors.
Box	Any type will do.		Must be large enough to allow stage movement. Needs apertures for input and output coupling of light.
Ceramic Optical Post	Any vendor.
Digital Microscope	Any vendor.		Optional.
Table Clamps	Thorlabs	CL5	Available from other vendors.
Bare Fiber clamps	Thorlabs	HFF003	Available from other vendors.
Table 1. Tapering Setup Materials.
As2S3 Optical Fiber	Fiber Optics Research Center of the Russian Academy of Sciences		Available from other vendors, such as CorActive.
Beavertail Cleaver	Fiber Network Tools	S-315	Available from other vendors. Hand cleaving or polishing fiber tips can also produce high quality fiber tips.
KimWipes	Kimberly-Clark Professional	34120	Available from other vendors.
Acetone, Isopropanol	Any vendor.
Table 2. Materials for Chalcogenide Fiber Preparation.
Pyrocam	Ophir Photonics	Pyrocam III Series	Any camera with sensitivity at pump wavelength will work.
Monochromator	Photon Technology International		A 100 line/mm grating was used. Any spectral measurement device will work (e.g. longpass filter).
CaF2 Lenses	Thorlabs	LB5922	Available from other vendors.
InAs Filter	Any vendor.		Available from other vendors.
Amplified InSb Detector	Hamamatsu	P4631-03	Available from other vendors.
Computer	Any vendor.
DAQ	National Instruments	USB X Series
Labview software for motorized stages	National Instruments		Optional. Custom program.
Labview software for collecting detector data	National Instruments		Optional. Custom program.
Assorted posts, optics mounts, bases, and forks
1" Gold mirrors	Any vendor.
Chopper and controller	Any vendor.	SRS Model SR540	Optional. Depends on detector being used.
Table 3. Materials for In-situ Tapering Procedure.