Partial Amplification of Octave-Spanning Supercontinuum in the Spectral Region of 1.5–2.2 μm

Abstract

Octave-spanning supercontinuum conversion in three different rare-earth doped fiber amplifiers have been investigated. Using an erbium amplifier, it turned out to increase the output power to 445 mW with a spectral width of 1250 nm. For a thulium amplifier, an average output power of 390 mW and a spectral width of 569 nm was obtained. Additionally, for holmium, the average output power was 724 mW with a spectral width of 450 nm. For all cases, the output pulses envelope did not exceed 0.72 ns.

1. Introduction

Supercontinuum generators (SC) have become widespread light sources in various scientific and industrial fields, for example, in telecommunications [1], hyperspectral microscopy [2], and medicine [3]. Currently, there is a need to increase the power density in the spectral regions of 1.5 μm, 1.9 μm and 2.1 μm, since there are promising applications of such sources for eye surgery [4], gas detection [5], respiration analysis [6], and also study of laser radiation effect on biological tissues [7,8].

Generally, SC generators are based on a pulsed master oscillator with a subsequent amplification of its radiation to increase the pulse’s energy, which would be sufficient for efficient conversion in a nonlinear medium [4]. For this purpose, various photonic-crystals [9], chalcogenide fiber [10], dispersion-decreasing fiber (DDF) [11] and high-nonlinear fiber [12,13] are used. Due to such nonlinear effects as self-phase modulation (SPM), solitons fission [14], modulation instability (MI), and many others, super-broadened radiation can be obtained.

Recently, there has been an increasing interest in SC sources based on all-fiber optical schemes, which are compact and robust in comparison with generators based on bulk components. One of the problems with all-fiber silica-based generators is the long-wavelength limitation of 2.5–2.8 µm since the silicon dioxide has a relatively high phonon energy level of ~1100 cm⁻¹ [15]. Therefore, based on an all-fiber optical scheme, one needs to blueshift the short-wavelength edge. From this point of view, laser sources emitting in the spectral range of 1 μm are preferable [16]. Another problem is the low power spectral density in the ranges beyond the pump laser wavelengths. The use of fiber amplifiers (FAs) could solve this problem.

One of the options for SC amplification is to use the effect of four-wave parametric amplification, which was implemented in gas-filled hollow fibers [17]. In addition, there is an interest in using FAs doped with various rare-earth elements (REE) (erbium, thulium, or holmium) at the output of the SC generator [18]. This approach leads to an all-fiber system design, without focusing on losses through the bulk optics. Indeed, the application of such a system was presented in [19], where the SC was amplified by using a thulium FA and the resulting radiation was directed into a second harmonic generator (SHG).

In this work, the usage of partial amplification of broadened radiation in the different spectral regions by FAs is presented. We used various silica-based REEs (erbium, thulium, or holmium) doped fibers as an amplifier’s active medium. It should be noted that a homemade dispersion decreasing fiber was used to obtain such a wide optical spectrum of SC. By varying the pump power of FAs doped with different REEs, it is possible to control temporal characteristics, spectral shape, and density of the output radiation. The amplification part of the SC spectrum allows obtaining various laser systems operating in the spectral range of 1–2 microns with subnanosecond pulse duration.

Therefore, the asymmetry appearing in the SC spectrum allows us to achieve greater energy in the necessary part of the optical spectrum. For example, such a scheme can be the basis for the development of a mobile source compatible with the endoscope, which would be used at the place of medical care. Another potential application of such a source could be the study of biological tissues in the human body. In particular, there are absorption lines of adipose tissues in the spectral region of 2.3 μm, so our data are suitable for this type of research. It is also possible to perform a spectral selection from both SC and amplified SC radiation with a sufficiently narrow spectral line due to the use of fiber Bragg gratings [20].

2. Materials and Methods

The experimental setup shown in Figure 1 includes a master oscillator (MO), a FA, and a nonlinear medium, which in total provided the SC generation. An all-fiber ytterbium-doped (Yb) laser operating in the passive mode-locking regime due to nonlinear polarization evolution was used as a MO. The laser cavity was formed by an active medium (multicomponent ytterbium-doped GTWave fiber) and a passive single-mode fiber (such elements as a fiber isolator, a polarization beam splitter, and a delay fiber). The total cavity length was about 200 m, which corresponds to a fundamental repetition rate of 1 MHz.

To increase the energy of the MO pulse, a ytterbium-doped fiber amplifier (YDFA) was used. Yb-doped GTWave fiber was used as an active medium. The core/cladding diameter of this fiber was 6/125 µm. The absorption at the pump wavelength (λ = 976 nm) for Yb-doped fiber was approximately 0.9 dB/m. The length of such a fiber was 8 m and was optimized for operation with MO. The pump (LD2) was a semiconductor laser diode emitting at a wavelength of 976 nm with an average output power of up to 8 W. To prevent back reflection of radiation into the system, an optical isolator (ISO) was placed between the MO and the amplifier. Subsequently, the obtained amplified radiation was directed into a nonlinear medium, where the generation of super-broadened radiation took place. We used the DDF with an average nonlinear coefficient γ = 10 W⁻¹·km⁻¹ as a nonlinear medium of the SC generator. The fiber diameter changed from 120 µm to 150 µm along with the cladding and from 6 µm to 9 µm at the core. The DDF fiber used in this work was produced in the course of joint research of GPI RAS and ICHPS RAS using the classical technique of taper-type fiber drawing while maintaining a smooth change in the core diameter along the length of the entire sample (here from 6 to 9 μm) [21,22]. In this fiber, the anomalous dispersion varied linearly along the fiber length from 0 ps/nm·km (d = 120 μm) to 11 ps/nm·km (d = 150 μm) at 1550 nm (Figure 2e). Radiation was launched from the side of a small core and detected from a large one.

Erbium, thulium, and holmium FAs were developed for partial amplification of SC radiation in different spectral regions (Figure 1-FA). The erbium-doped fiber amplifier (EDFA) operated in the spectral range of 1.5–1.6 μm. The semiconductor laser diode emitting at a wavelength of 976 nm with a maximum output power of up to 2.1 W was used as a pump source. A double-clad (DC) fiber doped with Er/Yb ions and 4 m long (iXblue) was used as the active medium of the FA. The numerical aperture of this fiber for the core and cladding was NA—0.19 and 0.46 with diameters of 6 μm and 125 μm, respectively. The absorption at a pump wavelength of 976 nm was 2.11 dB/m. The splice losses were estimated by the equipment and the value did not exceed 0.01 dB.

For the spectral region, a 1.8–2 μm thulium-doped fiber amplifier (TDFA) was assembled. As with the previous one, a semiconductor diode with an output power of up to 2.5 W, emitting at a wavelength of 793 nm, was used as a pump. A special DC fiber doped with thulium ions (nLightLiekki), 3.8 m long, was used as an active medium. The absorption at a wavelength of 790 nm was 4.8 dB/m, the core/cladding diameters were 10 µm and 125 µm. The core/cladding NA was 0.15 and 0.48, respectively. The splice losses did not exceed 0.01 dB.

For the longest wavelength spectral region of 2–2.2 μm, we have assembled the holmium-doped fiber amplifier (HDFA), which was pumped by an all-fiber CW ytterbium pump laser (PL) at a wavelength of 1130 nm with an output power of up to 3.2 W. The length of an active Ho-doped fiber was 3 m, and its absorption at a wavelength of 1125 nm was 12 dB/m. The fiber core diameter was 9 µm with a cladding diameter of 125 µm. The splice losses did not exceed 0.01 dB.

Optical spectra were controlled by listed optical spectrum analyzers: Avesta (spectral resolution 1 nm with a range of 900–2600 nm), HP (spectral resolution 0.1 nm with a range of 700–1700 nm), and Yokogawa (spectral resolution 0.1 nm with a range of 1200–2400 nm). The various photodetectors for different spectral regions 0.8–1.7 µm (rise time < 35 ps) and 1.5–2.5 µm (rise time < 100 ps), and a Tektronix oscilloscope (bandwidth up to 4 GHz) were used for pulses envelope measurement. The response time of this pulse measurement system is about 0.18 ns, which is about 2 times less than most measured pulse widths. Consequently, this allows us to consider the obtained temporal characteristics of the radiation to be modestly accurate. During the experiments of pulse envelope width measurements, a standard oscilloscope utility was used.

3. Results

As mentioned before, the Yb-doped fiber laser with a pulse duration of 0.26 ns (it was verified by a 26 GHz photodetector coupled to a 33 GHz oscilloscope), the energy of 5 nJ, and a repetition rate of 1 MHz were used as the MO. The lasing spectrum located in the range of 1063–1066 nm has a shape corresponding to a strongly chirped dissipative soliton [23]. In addition, the peak at a wavelength of 1120 nm matched to the 1st Stokes component was observed in the output laser spectrum. After amplification in the YDFA, it was possible to increase the average output power from 5 mW up to 800 mW. Then radiation was launched into the DDF, where, due to stimulated Raman scattering (SRS) in combination with MI and solitons fission [24,25,26,27], SCs with an octave width at −30 dB level is obtained. The average output power of the SC reached up to 340 mW with a pulse envelope duration of 0.35 ns (Figure 2).

Subsequently, the obtained SC radiation was directed into different FAs to realize amplification in the spectral regions of 1.5–1.6, 1.8–2.0, and 2–2.2 μm. Two spectrum analyzers (HP and Yokogawa) were used to estimate the full width of the SC radiation output spectrum in the range of 800–2400 nm with high resolution (0.5 nm).

In the first experiment, SC radiation was injected into an EDFA. The amplifier pump power was changed from 0 up to 2.1 W to track the radiation spectral composition changes. As can be seen in Figure 3a,c the maximum width of amplified SC optical spectra was 1260 nm (from 1040 to 2300 nm) at a signal level of −20 dB. The average output power increased from 340 mW (input SC power) up to 445 mW. The optical spectra contain two peaks in the spectral range of 1050–1065 nm, which correspond to two spectral peaks of the MO dissipative soliton. The power spectral density for obtained SC was 0.23 mW/nm. Due to the Er-doped fiber absorption in the spectral range of 1450–1490 nm and partial amplification in the range of 1530–1550 nm the significant changes in spectral shape were observed during the pump power changes (see Figure 3a,c).

We believe that partial amplification led to solitons formation in the 1530–1550 nm region and soliton self-frequency shift at higher pump power levels. However, we still can observe fine structure corresponding to the SRS effect in short wavelengths. There is also a peak in the 1111 nm (8998 cm⁻¹) region, which corresponds to the first Stokes component, and a hump in the 1250 nm region corresponds to the SC radiation. Based on the integral intensity of the entire spectrum, 90% of the whole energy consisted in the range of 1440–1820 nm at the maximum pump power. The temporal characteristics of the amplified radiation were also measured. Figure 3b,d shows the envelopes of the received pulses. The duration varied from 0.32 ns to 0.34 ns, which is comparable with the duration of the SC pulses envelope.

Using a TDFA, pumped by the laser diode at a wavelength of 793 nm allowed us to obtain partial amplification in the range of 1.8–2.0 μm. The maximum pump power level did not exceed 2.5 W. When using TDFA we observed absorption by a Tm-doped fiber occurred in the short-wavelength spectral region, and amplification occurred in the 2-μm region (from 1800 nm to 2000 nm) (Figure 4a,c). Here, the optical spectra are shown in the spectral range of 1200–2400 nm (resolution 0.1 nm), since the radiation in the short-wavelength region was dramatically suppressed. As can be seen from the obtained optical spectra, changing the amplifier pump power led to the changes in spectral shape in the range of 2300–2400 nm, which is associated with the conversion of radiation due to nonlinear effects. The shape of the output spectrum can be explained by the dominance of the SPM effect over the soliton frequency self-shift. In future studies, this fact will be verified by simulations. For a more accurate simulation, the measurement of dispersion and nonlinearity in active fibers will be carried out.

The estimated maximum width of the obtained spectra was 420 nm at a level of −20 dB since broadening does not occur beyond 2.5 μm (inset in Figure 4c). From the integral intensity, the energy in the spectral region of 1945–2012 nm was estimated as 85%. Using TDFA allowed us to achieve a maximum output average power of SC up to 390 mW. Figure 4b,d shows the envelopes of the amplified SC pulses, and their duration varied from 0.33 to 0.36 ns.

The third amplifier in this series of experiments was an HDFA. The pump was performed by the Yb-doped fiber laser with the output power varying from 0 to 3.2 W. Figure 5a,c shows the obtained optical spectra of SC radiation amplified in HDFA. As in the case of Tm-doped fiber, the Ho-doped fiber absorbs the short-wavelength radiation, generating SC radiation just in the region of 2.1 μm, where the spectrum width and intensity increased, depending on the used pump power (based on the integral intensity, the energy was 95%). The peak observed at a wavelength of 1750 nm refers to the unabsorbed part of the SC radiation since the absorption of Ho-doped fibers is in the regions of 1.1–1.2 μm and 1.8–2.1 µm, and the spectral dependence of the stimulated emission cross-section is 1.9–2.2 µm [28].

The peak width at a wavelength of 2100 nm increased from 125 nm at an average output power of 9.4 mW to 110 nm at a maximum average output power of 724 mW at a signal level of less than −10 dB. The output temporal parameters of the amplified radiation were obtained (Figure 5b,d), and the pulse envelope duration varied from 0.63 to 0.72 ns depending on the used pump power of the amplifier. In contrast to the two previous cases, the output spectrum width from HDFA was close to the gain spectrum of the active fiber. Together, the HDFA’s lower input signal and longer pulse duration resulted in HDFA’s pulse peak power being about twenty times lower. As a result, we did not achieve the gain (and peak power) level sufficient for significant spectral conversion.

Table 1. Broadband radiation output parameters.

	Pump, W	Active Fiber Length	Pout, mW	Δλ (−20 dB), nm	Brightest Spectral Range (−10 dB), nm	τ, ns
Initial SC	-	-	340	1260	1040–1166, 1957–2150	0.35
EDFA	2.1	4	445	460	1530–1800	0.34
TDFA	2.5	3.8	390	420	1930–2150	0.36
HDFA	3.2	3	724	180	2040–2150	0.72

shows all the obtained output radiation parameters when using different FAs in comparison with the parameters of SC radiation (the results are shown for the maximum values of the amplifiers’ pump powers). As one can see by using a simple optical scheme that consists of serial connected MO, FA1, DDF, and spectrally shifted FA2 we can obtain desirable subnanosecond irradiation in spectral ranges corresponding to the FA2 REE dopants. Additionally, Figure 6 shows a comparison of the obtained radiation spectra after using EDFA, TDFA, and HDFA. For EDFA, the maximum power spectral density was 6.4 mW/nm at 1540 nm, varying from 1.8 to 1 mW/nm in the spectral range 1600–1800 nm. For TDFA, the maximum power spectral density of 5.5 mW/nm was reached in the wavelength region of 1980 nm. The highest power spectral density 11 mW/nm was achieved at 2090 nm using TDFA.

In addition, we compared our results with existing works (

Table 2. Comparison of different SC generators and their partial amplification.

Octave SC
Ref.	Pump/Wavelength	Medium	Input Peak Power	Spectral Range (nm)	All-Fiber System
[29]	Nd:YAG microchip laser/1064 nm	graded-index MMFs	up to 35 kW	500–2500	no
[30]	Yb-doped fiber laser/1040 nm	graded-index MMFs	~30 kW	700–1700	no
[31]	1030 nm	PCF	300–400 W	800–1240	yes (highly coherent)
[32]	1016 nm	PCF	8.8 kW	350–2400	yes (used MFA)
Our system	Yb-doped fiber laser/1064 nm	DDF	about 1–2 kW	900–2320	yes (without MFA)
Amplification
[33]	Yb-doped fiber laser/1077 nm	DC Yb-doped fiber	about 6.5 kW	1037–2167	Yes
[18]	SC (460–1550)	Er-doped fiber	-	1500–1600	-
[19]	SC (900–2400)	Tm-doped fiber	-	1750–2200	Yes
Our system	SC (900–2320)	Er-doped fiber	about 1.3 kW	1520–1800	Yes
		Tm-doped fiber		1930–2150
		Ho-doped fiber		2040–2150

). The main difference between the system presented by us is the simplicity of design due to the use of only silica fibers. That is, our generator is all-fiber without the use of complex and expensive elements, such as bulk optics, mode field adapters (MFA), etc. After optimizing all the components of the generator, it is also possible to increase the energy of the pulsed radiation.

4. Conclusions

Partial amplification of the octave-spanning supercontinuum in the spectral region of 1–2.4 μm was investigated. FAs doped with various REEs (erbium, thulium, and holmium) were used for amplification in the spectral regions of 1.5 μm, 1.9 μm, and 2.1 μm. The maximum optical spectrum width of 1250 nm was obtained using EDFA, and the maximum output average power of 724 mW was obtained using HDFA. The temporal parameters of the amplified radiation in different spectral regions were also measured and the duration of the envelope did not exceed 0.73, 0.36, and 0.34 ns for HDFA, TDFA, and EDFA, respectively. This is comparable to the duration of the initial envelope of the SC pulse, which was 0.35 ns.

As a result, a simple and robust principal scheme was proposed, which allows for the generation of subnanosecond pulses in the spectral range of 1.5–2.2 μm. Based on the widespread availability of fiber optic components for the 1 µm range and oscillators, this approach may be preferable for some applications.