A special optical fiber pigtail with protruded tip and cladding light stripped design for high power pump laser

Photonstream is focusing on optical fiber precise polishing & AR coating over 10 years, manufacturing excellent optical fiber pigtail coupling laser diodes globally.

New Product Development Background:

Since the fast growing of 10KW class fiber laser market, it requests the higher power level of pump inside. Base on the current the 300~500W single module output, most of fiber laser companies are planning to develop next generation of 500~1000W pump laser. The key challenge is we have to keep 135um/200um core diameter for optic fiber tail coupling pump laser due to the higher brightness output request, which mean the higher laser energy density and the more cladding light will be coupled into this small optical fiber tail(Figure 1). If the cladding light was not stripped out in coupling terminal, it will be delivery to fiber combiner, which will cause the high temperature and fiber burning. In order to solve this problem, Photonstream R&D team are

working out a solution to strip the cladding light on the coupling fiber tip, this kind of new product is not only satisfying for 500~1000W pump laser coupling but also with excellent cladding light stripping.

Figure 1. pump laser optical road map

Product Overview：

The pump laser coupling optical fiber tail turnkey is consisting of a multiple mode fiber and a ceramic ferrule assembly. The fiber was inserted into the ferrule and was fixed by the UV epoxy, the super cleaning fiber tip is protruded out of the ferrule and with AR(anti-reflection) coating (Figure 2).

Figure 2.Pump optical laser fiber tail turnkey

Product Feature：

The protruding tip design fiber tail is focusing on high lever optical power coupling, with the etching on fiber side, the cladding light was stripped out in the ferrule inner hole and converted to heat energy to conduct to the laser package(Figure3). More detail feature is described in below:

1. Protruding & free epoxy of fiber tip design keep the endface and fiber side good cleanness, which is critical for high power laser.

2.  With the special optical polishing, the fiber tip surface is under super clean quality(no contamination, no scratch and no end chipping), this is the very good condition before AR coating.

   3. The coupling fiber tip is deposited high transmittance dielectric optical thin film coating, the

R<0.3% 900-1000nm AR coating design is able to cover both 915nm & 976nm chip wavelength. 

   4. The coating layer is with low absorbed and high laser damage threshold, which will prevent

very hot fiber or burning when hundreds of watts laser power was coupling to 135um/200um

core fiber. 

   5. The fiber cladding cylinder area is etched inside the ferrule which destroy the total reflection

condition between cladding and air, so the cladding light was stripped out effectively and convert

into thermal energy. 

   6. The etched cladding area was recoated with high transmittance UV epoxy, which help to fix the fiber inside the ferrule and the cladding light escape . 

   7. The ceramic ferrule is easily assembly to the pump laser copper package, which is good design for heat balance. 

   8. The other ferrule terminal was dispensed low Tg epoxy (flexible) to protect the fiber broken by external sudden bending.

Figure 3.The fiber tail Feature

In order to verify the performance of the new product, we carried out some tests as below:

Experiment I : (Figure 4. The different window of cladding light stripping)

As showing in figure 4, we use A DC fiber(dual cladding fiber), one terminal was coupled into 915nm fiber laser, manually we align some light into the cladding , the other fiber end connect to the power meter to really monitor the power change. Then strip 2 windows in fiber middle, window 1 without any change, window 2 with etching handling, then place two infrared sensor card under the two windows, when turn on the laser and we can find window 1 without any light while window 2 could see obvious cladding light stripped.

Left: window 1 no handling                      Right: window 2 with etching handling

Figure 4. The different window of cladding light stripping

If we paint the epoxy on window 2, then put on the ferrule tube, after the epoxy cured, turn on

the laser, use HUAWEI cell phone to take photos, we can see obviously cladding light transmission

from ferrule, then touch the ferrule tube with hand, which can feel a higher temperature.

Figure 5. window 2 with epoxy and ferrule

Experiment II:

We will choose 2 fiber pigtails with the same outer dimensions design (one fiber is normal fiber pigtail, the other fiber pigtail with cladding stripped ability), the two fiber tails are spliced to split laser source, both fiber tails have cladding light but only show cladding light stripped out in the ferrule area (Figure 6):

A. the fiber pigtail on the left without any technique doesn’t show cladding light

B. The end of fiber pigtail on the right with etching technique obviously show cladding light

spilled.

Figure 6. The two different fiber tail turnkey

Experiment III:

For further difference comparison of cladding light spilled, Photonstream R&D team also make 3 different samples for light transmission experiment, 3 samples are following: a. sample stripped cladding light with etching technique b. sample stripped cladding light with special epoxy c. sample without cladding light stripped . From the following Figure 7, we will get the sample with etching has obvious advantage in stripped cladding light.

Figure 7 Cladding Light Stripped Comparison 

Experiment IV: About the AR coating design for high power laser:

Photonstream R&D team are not only focusing on the stripped cladding light technology, but also concentrating on solving the hot fiber issue of high power laser coupling. Base on our study, we found there are two failure modes of AR coating fiber tip. 

The failure mode 1: the laser-induced permanent change on the coating layer only which is due to the lower Laser Damage Threshold coating material, this failure happens in the high power pulsed laser. 

The failure mode 2: Coated fiber tip burning or very hot due to highly laser absorption due to very bad deposition coating process or contamination reason, and this mainly happen in CW high power laser and this is the most failure mode type. Base on the study, Photonstream develop a consistent coating process to obtain the lower absorption and super cleaning coating layer surface. The fiber burning is solved and the fiber tip

temperature is keeping on around 50℃ for the 500W laser coupling(Figure 8) :

Figure 8. The AR coated fiber tip temperature when coupling 500W laser