1578 OPTICS LETTERS / Vol. 33, No. 14 / July 15, 2008

Watt-level Er-doped and Er–Pr-codoped ZBLANfiber amplifiers at the 2.7–2.8 �m

wavelength range

Xiushan Zhu1,2,* and Ravi Jain1

1Center for High Technology Materials, University of New Mexico, 1313 Goddard SE, Albuquerque,New Mexico 87106, USA

2Present address: College of Optical Sciences, University of Arizona, 1630 East University Boulevard, Tucson,Arizona 85721, USA

*Corresponding author: xszhu@email.arizona.edu

Received April 22, 2008; revised June 5, 2008; accepted June 9, 2008;posted June 16, 2008 (Doc. ID 95220); published July 11, 2008

Characteristics of diode-pumped Er-doped and Er–Pr-codoped ZBLAN fiber amplifiers were investigated atthe 2.7–2.8 �m wavelength range. An amplified signal of 4.6 W was obtained from the singly Er-doped am-plifier for an input signal of 110 mW, corresponding to a net gain of 16.2 dB. An amplified signal of 2.65 Wwas obtained from the Er–Pr-codoped amplifier for an input signal of 70 mW, corresponding to a net gain of15.8 dB. It is found that the gain bandwidth of the singly Er-doped amplifier is much narrower than that ofthe Er–Pr-codoped amplifier. Small gain, no gain, or even a loss for shorter-wavelength signals propagatingthrough the singly Er-doped amplifier should attribute for the large number residual populations in thelower laser level caused by the inefficient depopulation of the energy transfer upconversion processes.© 2008 Optical Society of America

OCIS codes: 060.2320, 140.3500, 140.3070, 140.4480.

Mid-IR oscillators, generally, including opticallypumped ion-doped crystal, glass, and optical fiber la-sers; electrically and optically pumped antimonide,IV–VI, and quantum cascade semiconductor lasers;optically pumped optical parameter oscillators anddifference frequency generators; and electricallypumped gas lasers have been paid much attention inthe past decades because of their many applicationsin spectroscopic sensors, IR countermeasures, lasersurgery, and high-efficiency pump sources for longer-wavelength oscillators. Recently, mid-IR fiber lasersare of rapidly increasing interest owing to their in-herent advantages, such as high optical conversionefficiency, simplicity and compactness, outstandingheat-dissipating capability, and excellent beam qual-ity. Various watt-level mid-IR fiber lasers [1–7] and a10 W 2.78 �m fiber laser [8] have been developed onrare-earth-doped ZBLAN fibers. A much higher out-put power of mid-IR fiber lasers is still technicallypossible. However, for an oscillator its critical speci-fications, such as spectral linewidth, power stability,or pulse duration, always degrade with its increasingoperation power. Therefore, it is usually not easy toreach the required performance in terms of line-width, stability, or pulse duration when the requiredpower level is very high. For instance, although anearly 10 W output had been obtained from the free-running Er-doped ZBLAN fiber laser [8], its spectrallinewidth and power stability were not acceptable forsome specific applications.

A mid-IR master oscillator–power amplifier(MOPA) is an effective solution to the problems ofZBLAN fiber lasers described above and is an easyapproach to achieve high-power or high-energysources with the required performance. As the power

0146-9592/08/141578-3/$15.00 ©

booster, therefore, mid-IR fiber amplifiers are of sig-nificant interest for the achievement of high-performance mid-IR MOPAs in the near future.

Er-doped ZBLAN mid-IR fiber amplifiers were ini-tially studied in the early 1990s under an intention toemploy them for ultralong distance optical communi-cation because of the predicted ultralow loss ofZBLAN fibers at the mid-IR range ��10−3 dB/km�.However, only low-concentration Er-doped ZBLAN fi-bers were investigated at very low power levels(input signal was approximately microwatts, outputsignal was approximately milliwatts) in the litera-ture [9–11]. In this Letter we present our investiga-tions on watt-level mid-IR fiber amplifiers at the2.7–2.8 �m wavelength range by use of a 4 m singlyEr-doped and a 9 m Er–Pr-codoped ZBLAN double-clad fiber, respectively.

The experimental setup for high-power diode-pumped mid-IR fiber amplifiers is depicted in Fig. 1.Broadly wavelength-tunable mid-IR fiber lasers at2.7–2.8 �m were employed as the signal sources[12,13]. A collimating-and-focusing system comprisedof two identical sapphire ball lenses with diameters

Fig. 1. (Color online) Experimental setup for high-powerdiode-pumped Er-doped and Er–Pr-codoped ZBLAN fiber

amplifiers.

2008 Optical Society of America

July 15, 2008 / Vol. 33, No. 14 / OPTICS LETTERS 1579

of 5 mm was used to couple the signal into the core ofthe angle-cleaved amplifier fiber. The amplifier fiberwas counterpropagating pumped by a 32 W 975 nmcollimated laser diode. The pump coupling lens was asapphire plano–convex lens (focal length=25 mm). Adichroic mirror with high reflectivity ��99.5% � at thelaser wavelength and high transmission ��95% � atthe pump wavelength was placed at a 45° angle of in-cidence to couple out the amplified signal. The outputsignal was purified by a long-wavelength pass filter�T�95% at. ��2.5 �m�. A thermal power meter anda CVI Digikrom 240 monochromator (focal length=0.5 m) were used to measure the power and thewavelength of the output, respectively.

Since different wavelength-tunable ranges of thesingly Er-doped and the Er–Pr-codoped ZBLANfibers have been observed [12,13], both custom-designed ZBLAN fibers (FiberLabs Inc.) were studiedin the amplification experiments. The concentrationof ions in the heavily Er-doped fiber was 6 mol%. Itscore diameter was 15 �m, and core NA was 0.13. Thedimension of the rectangular pumping inner claddingwas 200 �m�250 �m, and NA was 0.55. The concen-trations of Er and Pr ions in the codoped amplifier fi-ber were 2 mol. % and 0.5 mol. %, respectively. Itscore diameter was 13 �m, and the core NA was 0.16.The dimensions of the rectangular pumping innercladding were 100 �m�200 �m, and the NA was0.55. It is noted that both ends of the amplifier fiberswere angle cleaved ��10° �.

The characteristics of the singly Er-doped ZBLANfiber amplifier were first studied. When the pumppower was 5.4 W the spectra of the output of the am-plifier for a 110 mW signal of different wavelengths�2705–2805 nm� are shown in Fig. 2. When the sig-nal of 2705 nm was input, no output signal from theamplifier was detected except a laser peak around2782 nm [shown by the dashed black line inFig. 2(a)], which corresponds to the gain peak of thesingly Er-doped ZBLAN system. That indicates thatthe signal has been absorbed by the amplifier fiberand there is a loss—not a gain—for the signal of thiswavelength. As predicted, when the pump wasturned off the signal was detected because it was onlyattenuated by the propagation loss of the amplifier fi-ber. This interesting phenomenon confirms the inter-pretation in [12] that there is a strong reabsorptionin the singly Er-doped ZBLAN system and the reab-

Fig. 2. (Color online) Output spectra of the singlyEr-doped ZBLAN fiber amplifiers for 110 mW signals of dif-ferent wavelengths when the pump power is 5.4 W. (a) Sig-nal wavelengths are 2705, 2718, 2731, and 2736 nm; (b)signal wavelengths are 2744, 2752, 2761, 2770, 2778, 2788,

and 2805 nm.

sorption as well as the weakened population inver-sion induces the breakdown of lasing at shorterwavelengths [12]. For the signals of 2718, 2731, and2736 nm, as shown in Fig. 2(a), other than the signalpeak that was not fully amplified, there still existed aparasitic laser peak around 2782 nm. The amplifica-tion for the signals below 2740 nm cannot competeover the lasing at 2782 nm, because net gains forthese wavelengths are relatively small and there ex-ists a low-Q oscillating cavity between two couplinglenses (non-antireflection-coated) owing to theirFresnel reflections. When the signal wavelength waslonger than 2740 nm, the parasite laser peak disap-peared and amplified signals with narrow spectrallinewidth of �2 GHz were obtained, as shown inFig. 2(b). The output signal power and the net gain ofthe amplifier as a function of the pump power for110 mW input signals of 2744, 2788, and 2805 nmare shown in Figs. 3(a) and 3(b), respectively. Whenthe pump power was 20 W the amplified signal of2788 nm was 4.6 W, corresponding to a net gain of16.2 dB. The net gain at 2744 nm is larger than thatat 2805 nm when the pump power is smaller than8 W; however, it is smaller than that at 2805 nmwhen the pump power is larger than 8 W. This is be-cause the high-gain band shifts toward longer wave-lengths as the pump power is increased. It should beemphasized here that, in contrast to the unstableoutput of the singly Er-doped ZBLAN fiber laser athigh-power operation shown in [8], the high-powersignal from the fiber amplifier, shown by the uppercurve in Fig. 4, was very stable because it is just theconsequence of the stimulated amplification of thestable input signal, shown by the lower curve inFig. 4.

For the Er–Pr-codoped ZBLAN fiber amplifier thespectra of the output signals of different wavelengths�2703–2805 nm� when the launched signal powerwas 70 mW and the pump power was 3.35 W areshown in Fig. 5. No output signal of 2805 nm indi-cates that there is no gain at this wavelength in theEr–Pr-codoped system. In Fig. 5, the variation of theamplified signals from 2703 to 2793 nm is less than5%, which shows that the net gain of the Er–Pr-codoped fiber amplifier is almost flat over the 90 nmspectral range. The amplified signals and the netgains as a function of the pump power for 70 mW sig-nals of 2703, 2754, and 2793 nm are shown inFigs. 6(a) and 6(b), respectively. When the pumppower was 12.5 W, the amplified signal of 2754 nm

Fig. 3. (Color online) (a) Output power and (b) net gain asa function of pump power for the singly Er-doped ZBLANfiber amplifier for 110 mW signals of 2744 nm (square),

2788 nm (circle), and 2805 nm (diamond).

1580 OPTICS LETTERS / Vol. 33, No. 14 / July 15, 2008

was 2.65 W, corresponding to a net gain of 15.8 dB.Comparing the performance of the two mid-IR fiber

amplifiers we find that the amplification bandwidthof the singly Er-doped ZBLAN is much narrowerthan that of the Er–Pr-codoped ZBLAN. The high-gain bandwidth of the Er–Pr-codoped amplifier isabout 100 nm and its flatness is less than 5%. Thegain bandwidth of the singly Er-doped amplifier,however, not only shrinks but also shifts towardlonger wavelengths as the pump power is increased.And the profitable gain is significant only for signalsof 2740–2805 nm when the pump power is largerthan 5.4 W. The behavior differences between thetwo fiber amplifiers should arise from their differentpopulation inversion mechanisms. In the singlyEr-doped system the population inversion is realizedvia the energy transfer upconverison (ETU) pro-cesses, the interactions between two Er ions. In theEr–Pr-codoped system, however, the population in-version is realized via the energy transfer (ET) pro-cesses, the interactions between Er and Pr ions. Ashas been explained in [12,13], owing to the inefficientpopulation depletion of the ETU process in the singlyEr-doped system, a large number of populations re-side in the lower laser level and most of them stay atthe lower Stark splittings according to the Maxwell–Boltzmann distribution. The residual populations in-creasing with the pump power result in a weak popu-lation inversion but a strong reabsorption for shorter-

Fig. 4. (Color online) Temporal stability of the 110 mW in-put signal (lower curve) and the 4.6 W output signal (uppercurve) of the singly Er-doped ZBLAN fiber amplifier.

Fig. 5. (Color online) Output spectra of the Er–Pr-codopedZBLAN fiber amplifier for 70 mW signals of different wave-lengths when the pump is 3.35 W. Signal wavelengths are2703, 2709, 2719, 2730, 2738, 2754, 2763, 2772, 2789, and2793 nm.

wavelength signals. As a result, the gain of the blueband of the singly Er-doped system goes to zero oreven a negative value when the pump power is high.In the Er–Pr-codoped system, however, the depletionof the ET processes is very efficient and only a smallnumber populations reside in the lower laser level;therefore, a weak population inversion will not occureven at a high pump level and consequently the am-plification bandwith does not shrink with increasingpump power.

In summary, we have demonstrated two watt-leveldiode-pumped mid-IR fiber amplifiers by use of a 4 msingly Er-doped and a 9 m Er–Pr-codoped ZBLANdouble-clad fiber. A nearly 5 W amplified signal and a16.2 dB net gain were obtained. Higher output andlarger gain are expected if a more powerful laser di-ode is used. We also find that the amplification band-width of the singly Er-doped ZBLAN fiber amplifieris not as broad as that of the Er–Pr-codoped ZBLANfiber amplifier and shifts toward longer wavelengthswhen the pump power is increased.

This work was financially supported by NASA, theAir Force Office of Scientific Research, and theOptoelectronics Industry Development Association.

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Fig. 6. (Color online) (a) Output power and (b) net gainas a function of pump power for the diode-pumpedEr–Pr-codoped ZBLAN fiber amplifier for 70 mW signals of2703 nm (circle), 2754 nm (diamond), and 2793 nm(square).