Type: Original Research Paper / Tipo: Artículo Original de Investigación Section: Optoelectronics / Sección: Optoelectrónica
Development of 3.6 mW Er- doped C-band super fluorescent fiber
source pumped at 980 nm
P. Nageswara Rao
(1,*), S K Shrivastava
(2)1. Research Centre Imarat, Hyderabad, AP– 500069, India.
2. Department of Physics, Bundelkhand University, Jhansi, UP - 284128, India.
(*) Email: [email protected]
Received / Recibido: 05/02/2013. Revised / Revisado: 21/03/2013. Accepted / Aceptado: 25/03/2013. DOI: http://dx.doi.org/10.7149/OPA.46.3.249
ABSTRACT:
A 3.6mW single pass backward signal superfluorescent fiber source having a spectral width of 27.417nm was developed at a pump wavelength of 980.5nm and presented the experimental results along with non-flattened ASE spectrum. The fiber source could have direct applications in fiber sensors like fiber gyroscope or, in general, in any optical sensors in which a broad spectral band and high power source are required.
Key words: Asymmetric Apodization, Complex-Pupil Filter, Sparrow Resolution Limit, Superresolution, Two-Point Resolution, Degree of Coherence, Intensity Ratio.
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1.
Introduction
The 1550nm Er-doped SFS in single pass backward configuration pumped at 980 nm integrated with fiber gyro scope has been studied extensively coupling 0.3 mW power to 500 m gyro coil and 1.0 mW power to 10 km gyro coil and achieved a short term resolution of 0.50/h in FOG [1]. Also a fiber optic gyro scope has been developed using SLD as light source
configuration operating at 1060 nm wavelength and integrated with fiber optic gyro scope [7-9].
The minimum detectable rotation rate of fiber gyro scope (FOG) primarily depends on wavelength stability of light source [10]. Coherence related excess noise of FOG which depends on the bandwidth of light source [11,12] and the resolution of FOG is proportion to the coherence time of light source [13,14]. The minimum 3-dB bandwidth of light source is to be 10-nm to eliminate coherent errors that results from Rayleigh backscattering [10], polarization cross coupling [11,12] and the optical Kerr effect [13,15]. Further more, the FOG light source should offer output power >1 mW for low shot noise performance of FOG [14]. The semiconductor superfluorescent light sources such as ELEDs and SLDs have been the sources for FOG [9,5,6]. The ELEDs and LEDs have high temporal coherence and failed to satisfy the wavelength stability requirement of light source for FOG [2,16]. These sources have short life time and they incur high coupling loss when they are coupled into FOG input single mode fiber [17,18]. But they are low cost and offer a very large bandwidth (>60 nm). when ELEDs and SLDs are driven at high power, the reflectance from the facets and the pigtails to the optical components produce residual coherence peaks which distort the interference signal from FOG and thereby limit the resolution of FOG. In order to address these concerns, Nd3+ fiber
sources have been developed at 1060nm wavelength [7-9,19]. Although the spectrum of Nd3+ fiber source is not as broad as that of SLDs, the useful coupled powers are typically >1 mW. Additionally, the Nd3+ fiber sources do not have
residual coherence spikes which are common in high power SLDs. The Nd3+ fiber sources are
sensitive to proton and gamma radiation. The change to Er3+ fiber sources from Nd3+
fiber sources appears to be the most promising choice of the light source for FOG due to several advantages. These include the operation of source in a low–loss wavelength region (1550 nm) of silica fiber, broader bandwidth and increased resistance to proton and gamma radiations at 1550 nm. The 1550 nm Er3+-doped
fiber sources were developed to elevate the above problems [20]. The co-dopents like
aluminum in erbium doped fiber has expanded the bandwidth of source >10 nm.
The erbium doped fiber can be configured into four basic kinds and their derivatives. These are single–pass forward signal (SPFS) SFS, single–pass backward signal (SPBS) SFS, double– pass forward signal (DPFS) SFS, double–pass backward signal (DPBS) SFS and fibre amplifier source (FAS) [21-23]. The characteristics of the erbium doped SFS strongly depends on which configuration is employed [2,24-26]. The Er3+
-ion in erbium doped fiber can be excited at 514.5 nm, 650-680 nm, 807 nm, 980 nm and 1480 nm pump wave lengths [21,27-29]. 980 nm pump wavelength is preferred to pump Er-doped SFSs because this wavelength provides relatively high gain, low noise and very low excited state absorption (ESA) [22,30]. Laming et al brought out the superiority of 980 nm pump wavelength than 1480 nm [31], the temperature degradation of gain in erbium doped fibers at 980nm and 1480 nm pump wavelengths were very well studied and reported by Dagenais et al [23]. The erbium doped fibers can also be configured in to resonant Er-doped fiber sources [32-37] and non-resonant Er-doped fiber for FOG [21,23-30,38]. The non-resonant Er3+ doped fiber
sources have shown capable of large spectral width with long life (about 15 years). The resonant Er3+-fiber sources have the highest
ratio of output power to pump power and non-resonant double pass SFSs offering intermediate value and single pass non-resonant SFSs giving the lowest value. Wang et al compared the efficiency and output powers of SBP, DPF and DPB SFSs configurations [38]. Lion et al compared the characteristics of DPB and DPF superfluorescent fiber sources [25]. Dulling et al reported the performance of DPB SFS pumped near 980 nm [39]. Several fiber sources using DPB configuration have been studied (see [38] and references therein, and [40-44]).
2.
Operation of superfluorescent
fiber sources
which occur in Er–doped fiber oscillator [45-49]. The erbium in the doped fiber acts as three level laser system. The three energy levels of interest for Er3+ pumped at 980 nm is high pump level
(4I11/2) and the two levels 4I15/2, 4I13/2
corresponding to the relevant laser transitions around 1530 nm [21]. Erbium in glass host, the surrounding crystalline field causes a Stark splitting Er3+ orbital’s and site-to-site variation
of the field due to the amorphous nature of glass results in an inhomogeneous broadening of the transition. The eight fold and seven fold Stark splitting of the ground 4I15/2 and upper 4I13/2
levels respectively combined with the effect inhomogeneous broadening makes Er3+ a
complicated multilevel laser system. The Stark split components are generally separated by 10-100 cm-1 [44]. When Er-doped fiber is pumped
at 980 nm wavelength, the electrons in ground state (4I15/2) absorb pump power and go to 4I11/2
state which has a short life of about 7 µsec. The exited electron from 4I11/2 relaxes
non-radiatively to upper–laser state (4I13/2) which
has a long life time of about 10 msec. The amplified spontaneous emission occurs near 1550nm when electrons decay from 4I13/2 which
is upper laser state to the lower laser state 4I15/2
which is a ground state. Both the upper laser state and ground state are Stark split manifolds
with homogeneous and inhomogeneous
broadening and both are occupied according to Boltzmann statistics. The entire emission spectrum spans from 1525 to 1565nm(C-band).
2.1. SPB Er3+-doped SFS for FOG
Although various SFS configurations have been proposed and utilized [21,38], single pass backward signal proved most useful The SPBS configuration has been widely used in an FOG because of its simple configuration and stable performance. In all other SFS configurations, accidental resonant lasing is a constant threat because optical components and FOG itself create feedback to the SFS. For this reason, this article presents solely for the SPBS super fluorescent fiber source configuration. The stable mean wavelength operation for SBP SFSs when pumped near 980 nm was first studied by Wysocki et al [21,50,51] and experimentally analyzed by Hall et al [27]. Trommer et al constructed three axis FOGs using single pass
backward SFS [4-6]. Iwatsuki tested the single pass backward SFS for excess noise [1]. The line width of SPB SFSs have been modified using gratings [52-56,9]. Fesler et al developed 12 nm band width 1060 nm Nd3+SPB SFS pumped at
815 nm and tested the FOG with coupled power of 1.5 mW [8,9]. The paper presents the development of 3.6 mW Er3+-doped C-band
superfluorescent fiber pumped at 980 nm in single pass backward configuration using commercially available optical components along with the experimental results. Figure 1 shows the configuration of SPBS super fluorescent fiber source.
This configuration uses pigtailed pump laser diode, wavelength division multiplexing (WDM) coupler, erbium doped fiber (EDF) and fiber isolator.
The fluorescent rare earth doped fibre source (SFS) output is simply amplified spontaneous emission(ASE) generated by the inverted Er3+
ions 4I13/2–4I15/2 transition as the three level laser
system. These two states are separated by an energy difference ( 0.8 ev) at or below room temperature. Thus generated forward and backward ASEs in the erbium doped fiber, are confines by the EDF core in both the forward and backward directions, are amplified as they travel along the fiber. The SFS does not have a resonator; the spectral emission covers a broad spectral band because of stark splitting of erbium manifolds, typically a few tens of nanometers.
Fig. 1. Single pass backward signal SFS configuration.
3.
Measurements, experimental setup
and results
pigtailed pump laser diode at 500mA input current is tabulated in Table I.
The erbium doped fibre procured from M/s OFS, USA has the Absorption and gain characteristics as shown in Fig. 3.
The EDF had a 1.55 µm core radius and numerical aperture of 0.223 with an erbium concentration of 10.62×1024 m-3.
TABLE I
Performance of pump laser diode at mean wavelength of 980.5 nm.
Threshold Current 28.7mA
Slope Efficiency 0.5574 mW/mA
Peak Wavelength 980.5 nm
Operating Temperature 25ºC
Power at 500 mA 256 mW
Fig. 2. Mean wavelength of pump laser diode measured using optical spectrum analyzer (OSA, ANDO model AQ6317C).
Fig. 3. Absorption and gain characteristics of MP980 erbium doped fibre from M/s OFS, USA.
We measured ASE power as a function of pump powers in terms of pump current levels for erbium doped fiber of length 35 meters. The experimental results of these measurements are shown in Fig. 4 and Table II.
TABLE II
Pump power as a function of LD input current.
LD Current (mA)
LD Pigtailed Power
(mW)
LD Voltage (V)
0.1604 0.037563 1.0285
29.964701 0.004961 1.33143
49.961498 0.855444 1.36411
99.976402 7.44668 1.44048
149.983795 15.0585 1.51621
199.998703 22.5853 1.59173
249.998398 30.112 1.66703
300.013306 37.2136 1.74234
350.020691 44.017399 1.81753
400.028015 51.246498 1.89305
410.033997 52.649799 1.908
450.035309 58.092899 1.96835
500.050201 64.173897 2.04408
Fig. 4: EDF output power as a function of input pump power at 980 nm. Pump wavelength for EDF length of 35 meters.
4.
The experimental setup of SPBS
super fluorescent fiber source
Fig. 5: The experimental setup of SPBS super fluorescent fiber source.
coiled to a diameter of 60 mm. The MP980erbium doped fiber was coiled to the diameter of 35mm. This results the actual placement of SFS components in SFS package leading to physical size of the device.
Presently, the commercial FOGs weigh 200-250 gms with a physical size of 100 mm × 100 mm × 50 mm.
The experimental configuration was realized with minimum optical components for SFS so that the weight is restricted to below 100 grams and the size to 100 mm × 100 mm × 15 mm. We understand this weight and size of SFS attract considerations as an optical source for applications like FOG and other fibre optic sensors.
The experimental SPBS super fluorescent fibre source was implemented by using the procured components. The EDF is from M/s OFS, USA, (EDF-MP980, Lot No: MP2A4201), the 980 nm pump laser diode is from M/s Powernetix, USA (9000 series, 5A), the WDM coupler (M/s Oplink Communications, Inc., USA, 980/1550 nm, FWDM59 Series), fiber isolator (M/s Oplink, 1550nm Single Stage Fiber Optical Isolator, 250 µm, Item # OISS511111).
The MP980 erbium doped fibre was doped with erbium and co doped with aluminum. We made several measurements on different lengths of erbium doped fiber.
At a length of 35 meters of EDF, the mean wavelength of backward signal ASE varied only less than one nanometer.
This is the length we finalized for EDF in the SPBS super fluorescent fiber source. The 35 meters length EDF was longer than optimum for generating backward ASE power for a pump powers considered. This extended length served three main purposes. First, the length of EDF beyond optimal produced forward ASE
absorption that reduced round–trip gain and prevented resonant lasing. Second, this EDF length guaranteed the absorption of more than 99% of the pump power across most of the pump band around 980 nm. Finally, since the backward ASE signal was the desired output, reduction of the forward ASE signal was actually an advantage. Any photon emitted in the forward direction of the EDF was not available for emission in the backward direction EDF. One end of the EDF was polished at a 15º angle to reduce reflections to an estimated level of -60 dB.
The pigtailed pump laser diode, WDM coupler, erbium doped fiber and fiber isolator were carefully spliced to a splice loss of less than 0.02 dBm. The fiber isolator was introduced in the SPBS SFS to avoid the presence of pump wavelength in the backward ASE spectrum of fiber source.
A 980 nm pigtailed laser diode was used as a pump source. The pump light was launched into the EDF through WDM fiber coupler. The pumped EDF produces ASE in both the forward and backward directions. The backward ASE is directed by a WDM coupler to a fiber pigtailed isolator. The spectrum of the source output was measured using an optical spectrum analyzer (OSA).
In this work the super fluorescent output signal was measured in backward direction using an OSA (ANDO model AQ6317C). The output power in the forward direction was about one nanowatt.
Fig. 6. The ASE spectrum of SPBS super fluorescent fiber source at 400 mA LD input current or 51.246498 mW input pump power measured using optical spectral analyses (OSA, ANDO model AQ6317C). The ASE output power is 3.628 mW and the spectral width is 27.417 nm.
The novelty of the experimental results and the device is that we have achieved 3.6 mW output power and 27.417 nm band width for Er3+ doped SFS in single pass backward
configuration pumped at 980 nm fabricated with all commercially available optical components. the SFS package occupies volume of 100 mm × 100 mm × 25 mm which is the smallest ever reported and smaller than commercially available FOG.